[ValueTracking] make dominator tree requirement explicit for isKnownNonNullFromDominatingCondition(); NFCI

I don't think this hole is currently exposed, but I crashed regression tests for
jump-threading and loop-vectorize after I added calls to isKnownNonNullAt() in
InstSimplify as part of trying to solve PR28430:
https://llvm.org/bugs/show_bug.cgi?id=28430

That's because they call into value tracking with a context instruction, but no
other parts of the query structure filled in.

For more background, see the discussion in:
https://reviews.llvm.org/D27855

llvm-svn: 290786

llvm/include/llvm/Analysis/ValueTracking.h

@@ -308,12 +308,12 @@ template <typename T> class ArrayRef;
   bool isKnownNonNull(const Value *V);
 
   /// Return true if this pointer couldn't possibly be null. If the context
-  /// instruction is specified, perform context-sensitive analysis and return
-  /// true if the pointer couldn't possibly be null at the specified
-  /// instruction.
+  /// instruction and dominator tree are specified, perform context-sensitive
+  /// analysis and return true if the pointer couldn't possibly be null at the
+  /// specified instruction.
   bool isKnownNonNullAt(const Value *V,
                         const Instruction *CtxI = nullptr,
-                        const DominatorTree *DT  = nullptr);
+                        const DominatorTree *DT = nullptr);
 
   /// Return true if it is valid to use the assumptions provided by an
   /// assume intrinsic, I, at the point in the control-flow identified by the

llvm/lib/Analysis/ValueTracking.cpp

@@ -3368,6 +3368,8 @@ static bool isKnownNonNullFromDominatingCondition(const Value *V,
                                                   const DominatorTree *DT) {
   assert(V->getType()->isPointerTy() && "V must be pointer type");
   assert(!isa<ConstantData>(V) && "Did not expect ConstantPointerNull");
+  assert(CtxI && "Context instruction required for analysis");
+  assert(DT && "Dominator tree required for analysis");
 
   unsigned NumUsesExplored = 0;
   for (auto *U : V->users()) {
@@ -3410,7 +3412,10 @@ bool llvm::isKnownNonNullAt(const Value *V, const Instruction *CtxI,
   if (isKnownNonNull(V))
     return true;
 
-  return CtxI ? ::isKnownNonNullFromDominatingCondition(V, CtxI, DT) : false;
+  if (!CtxI || !DT)
+    return false;
+
+  return ::isKnownNonNullFromDominatingCondition(V, CtxI, DT);
 }
 
 OverflowResult llvm::computeOverflowForUnsignedMul(const Value *LHS,