[analyzer][Liveness][NFC] Remove an unneeded pass to collect variables that appear in an assignment

Suppose you stumble across a DeclRefExpr in the AST, that references a VarDecl.
How would you know that that variable is written in the containing statement, or
not? One trick would be to ascend the AST through Stmt::getParent, and see
whether the variable appears on the left hand side of the assignment.

Liveness does something similar, but instead of ascending the AST, it descends
into it with a StmtVisitor, and after finding an assignment, it notes that the
LHS appears in the context of an assignemnt. However, as [1] demonstrates, the
analysis isn't ran on the AST of an entire function, but rather on CFG, where
the order of the statements, visited in order, would make it impossible to know
this information by descending.

void f() {
  int i;

  i = 5;
}

`-FunctionDecl 0x55a6e1b070b8 <test.cpp:1:1, line:5:1> line:1:6 f 'void ()'
  `-CompoundStmt 0x55a6e1b07298 <col:10, line:5:1>
    |-DeclStmt 0x55a6e1b07220 <line:2:3, col:8>
    | `-VarDecl 0x55a6e1b071b8 <col:3, col:7> col:7 used i 'int'
    `-BinaryOperator 0x55a6e1b07278 <line:4:3, col:7> 'int' lvalue '='
      |-DeclRefExpr 0x55a6e1b07238 <col:3> 'int' lvalue Var 0x55a6e1b071b8 'i' 'int'
      `-IntegerLiteral 0x55a6e1b07258 <col:7> 'int' 5

void f()
 [B2 (ENTRY)]
   Succs (1): B1

 [B1]
   1: int i;
   2: 5
   3: i
   4: [B1.3] = [B1.2]
   Preds (1): B2
   Succs (1): B0

 [B0 (EXIT)]
   Preds (1): B1

You can see that the arguments (rightfully so, they need to be evaluated first)
precede the assignment operator. For this reason, Liveness implemented a pass to
scan the CFG and note which variables appear in an assignment.

BUT.

This problem only exists if we traverse a CFGBlock in order. And Liveness in
fact does it reverse order. So a distinct pass is indeed unnecessary, we can
note the appearance of the assignment by the time we reach the variable.

[1] http://lists.llvm.org/pipermail/cfe-dev/2020-July/066330.html

Differential Revision: https://reviews.llvm.org/D87518

clang/include/clang/Analysis/CFG.h

@@ -1307,6 +1307,12 @@ public:
 
   iterator nodes_begin() { return iterator(Blocks.begin()); }
   iterator nodes_end() { return iterator(Blocks.end()); }
+
+  llvm::iterator_range<iterator> nodes() { return {begin(), end()}; }
+  llvm::iterator_range<const_iterator> const_nodes() const {
+    return {begin(), end()};
+  }
+
   const_iterator nodes_begin() const { return const_iterator(Blocks.begin()); }
   const_iterator nodes_end() const { return const_iterator(Blocks.end()); }
 
@@ -1315,6 +1321,13 @@ public:
   const_reverse_iterator    rbegin()      const    { return Blocks.rbegin(); }
   const_reverse_iterator    rend()        const    { return Blocks.rend(); }
 
+  llvm::iterator_range<reverse_iterator> reverse_nodes() {
+    return {rbegin(), rend()};
+  }
+  llvm::iterator_range<const_reverse_iterator> const_reverse_nodes() const {
+    return {rbegin(), rend()};
+  }
+
   CFGBlock &                getEntry()             { return *Entry; }
   const CFGBlock &          getEntry()    const    { return *Entry; }
   CFGBlock &                getExit()              { return *Exit; }

clang/lib/Analysis/LiveVariables.cpp

@@ -325,6 +325,11 @@ static bool writeShouldKill(const VarDecl *VD) {
 }
 
 void TransferFunctions::VisitBinaryOperator(BinaryOperator *B) {
+  if (LV.killAtAssign && B->getOpcode() == BO_Assign) {
+    if (const auto *DR = dyn_cast<DeclRefExpr>(B->getLHS()->IgnoreParens())) {
+      LV.inAssignment[DR] = 1;
+    }
+  }
   if (B->isAssignmentOp()) {
     if (!LV.killAtAssign)
       return;
@@ -513,29 +518,8 @@ LiveVariables::computeLiveness(AnalysisDeclContext &AC, bool killAtAssign) {
   llvm::BitVector everAnalyzedBlock(cfg->getNumBlockIDs());
 
   // FIXME: we should enqueue using post order.
-  for (CFG::const_iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it) {
-    const CFGBlock *block = *it;
-    worklist.enqueueBlock(block);
-
-    // FIXME: Scan for DeclRefExprs using in the LHS of an assignment.
-    // We need to do this because we lack context in the reverse analysis
-    // to determine if a DeclRefExpr appears in such a context, and thus
-    // doesn't constitute a "use".
-    if (killAtAssign)
-      for (CFGBlock::const_iterator bi = block->begin(), be = block->end();
-           bi != be; ++bi) {
-        if (Optional<CFGStmt> cs = bi->getAs<CFGStmt>()) {
-          const Stmt* stmt = cs->getStmt();
-          if (const auto *BO = dyn_cast<BinaryOperator>(stmt)) {
-            if (BO->getOpcode() == BO_Assign) {
-              if (const auto *DR =
-                    dyn_cast<DeclRefExpr>(BO->getLHS()->IgnoreParens())) {
-                LV->inAssignment[DR] = 1;
-              }
-            }
-          }
-        }
-      }
+  for (const CFGBlock *B : cfg->nodes()) {
+    worklist.enqueueBlock(B);
   }
 
   while (const CFGBlock *block = worklist.dequeue()) {