[NFCI][SimplifyCFG] Add basic scaffolding for gradually making the pass DomTree-aware

Two observations:
1. Unavailability of DomTree makes it impossible to make
  `FoldBranchToCommonDest()` transform in certain cases,
   where the successor is dominated by predecessor,
   because we then don't have PHI's, and can't recreate them,
   well, without handrolling 'is dominated by' check,
   which doesn't really look like a great solution to me.
2. Avoiding invalidating DomTree in SimplifyCFG will
   decrease the number of `Dominator Tree Construction` by 5
   (from 28 now, i.e. -18%) in `-O3` old-pm pipeline
   (as per `llvm/test/Other/opt-O3-pipeline.ll`)
   This might or might not be beneficial for compile time.

So the plan is to make SimplifyCFG preserve DomTree, and then
eventually make DomTree fully required and preserved by the pass.

Now, SimplifyCFG is ~7KLOC. I don't think it will be nice
to do all this uplifting in a single mega-commit,
nor would it be possible to review it in any meaningful way.

But, i believe, it should be possible to do this in smaller steps,
introducing the new behavior, in an optional way, off-by-default,
opt-in option, and gradually fixing transforms one-by-one
and adding the flag to appropriate test coverage.

Then, eventually, the default should be flipped,
and eventually^2 the flag removed.

And that is what is happening here - when the new off-by-default option
is specified, DomTree is required and is claimed to be preserved,
and SimplifyCFG-internal assertions verify that the DomTree is still OK.

llvm/include/llvm/Transforms/Utils/Local.h

@@ -30,6 +30,7 @@
 #include "llvm/IR/Value.h"
 #include "llvm/IR/ValueHandle.h"
 #include "llvm/Support/Casting.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Transforms/Utils/SimplifyCFGOptions.h"
 #include <cstdint>
 #include <limits>
@@ -186,6 +187,7 @@ bool EliminateDuplicatePHINodes(BasicBlock *BB);
 /// It returns true if a modification was made, possibly deleting the basic
 /// block that was pointed to. LoopHeaders is an optional input parameter
 /// providing the set of loop headers that SimplifyCFG should not eliminate.
+extern cl::opt<bool> RequireAndPreserveDomTree;
 bool simplifyCFG(BasicBlock *BB, const TargetTransformInfo &TTI,
                  const SimplifyCFGOptions &Options = {},
                  SmallPtrSetImpl<BasicBlock *> *LoopHeaders = nullptr);

llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp

@@ -31,6 +31,7 @@
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/Dominators.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Module.h"
@@ -191,8 +192,9 @@ static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
   return Changed;
 }
 
-static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
-                                const SimplifyCFGOptions &Options) {
+static bool simplifyFunctionCFGImpl(Function &F, const TargetTransformInfo &TTI,
+                                    DominatorTree *DT,
+                                    const SimplifyCFGOptions &Options) {
   bool EverChanged = removeUnreachableBlocks(F);
   EverChanged |= mergeEmptyReturnBlocks(F);
   EverChanged |= iterativelySimplifyCFG(F, TTI, Options);
@@ -216,6 +218,22 @@ static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
   return true;
 }
 
+static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
+                                DominatorTree *DT,
+                                const SimplifyCFGOptions &Options) {
+  assert((!RequireAndPreserveDomTree ||
+          (DT && DT->verify(DominatorTree::VerificationLevel::Full))) &&
+         "Original domtree is invalid?");
+
+  bool Changed = simplifyFunctionCFGImpl(F, TTI, DT, Options);
+
+  assert((!RequireAndPreserveDomTree ||
+          (DT && DT->verify(DominatorTree::VerificationLevel::Full))) &&
+         "Failed to maintain validity of domtree!");
+
+  return Changed;
+}
+
 // Command-line settings override compile-time settings.
 static void applyCommandLineOverridesToOptions(SimplifyCFGOptions &Options) {
   if (UserBonusInstThreshold.getNumOccurrences())
@@ -245,14 +263,19 @@ PreservedAnalyses SimplifyCFGPass::run(Function &F,
                                        FunctionAnalysisManager &AM) {
   auto &TTI = AM.getResult<TargetIRAnalysis>(F);
   Options.AC = &AM.getResult<AssumptionAnalysis>(F);
+  DominatorTree *DT = nullptr;
+  if (RequireAndPreserveDomTree)
+    DT = &AM.getResult<DominatorTreeAnalysis>(F);
   if (F.hasFnAttribute(Attribute::OptForFuzzing)) {
     Options.setSimplifyCondBranch(false).setFoldTwoEntryPHINode(false);
   } else {
     Options.setSimplifyCondBranch(true).setFoldTwoEntryPHINode(true);
   }
-  if (!simplifyFunctionCFG(F, TTI, Options))
+  if (!simplifyFunctionCFG(F, TTI, DT, Options))
     return PreservedAnalyses::all();
   PreservedAnalyses PA;
+  if (RequireAndPreserveDomTree)
+    PA.preserve<DominatorTreeAnalysis>();
   PA.preserve<GlobalsAA>();
   return PA;
 }
@@ -278,6 +301,9 @@ struct CFGSimplifyPass : public FunctionPass {
       return false;
 
     Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
+    DominatorTree *DT = nullptr;
+    if (RequireAndPreserveDomTree)
+      DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
     if (F.hasFnAttribute(Attribute::OptForFuzzing)) {
       Options.setSimplifyCondBranch(false)
              .setFoldTwoEntryPHINode(false);
@@ -287,11 +313,15 @@ struct CFGSimplifyPass : public FunctionPass {
     }
 
     auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
-    return simplifyFunctionCFG(F, TTI, Options);
+    return simplifyFunctionCFG(F, TTI, DT, Options);
   }
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.addRequired<AssumptionCacheTracker>();
+    if (RequireAndPreserveDomTree)
+      AU.addRequired<DominatorTreeWrapperPass>();
     AU.addRequired<TargetTransformInfoWrapperPass>();
+    if (RequireAndPreserveDomTree)
+      AU.addPreserved<DominatorTreeWrapperPass>();
     AU.addPreserved<GlobalsAAWrapperPass>();
   }
 };

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -87,6 +87,12 @@ using namespace PatternMatch;
 
 #define DEBUG_TYPE "simplifycfg"
 
+cl::opt<bool> llvm::RequireAndPreserveDomTree(
+    "simplifycfg-require-and-preserve-domtree", cl::Hidden, cl::ZeroOrMore,
+    cl::init(false),
+    cl::desc("Temorary development switch used to gradually uplift SimplifyCFG "
+             "into preserving DomTree,"));
+
 // Chosen as 2 so as to be cheap, but still to have enough power to fold
 // a select, so the "clamp" idiom (of a min followed by a max) will be caught.
 // To catch this, we need to fold a compare and a select, hence '2' being the