[RS4GC] Lazily set changed flag when folding single entry phis

The function FoldSingleEntryPHINodes() is changed to return if
it has changed IR or not. This return value is used by RS4GC to
set the MadeChange flag respectively.

Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D93810

llvm/lib/Transforms/Utils/BasicBlockUtils.cpp

@@ -136,9 +136,10 @@ bool llvm::EliminateUnreachableBlocks(Function &F, DomTreeUpdater *DTU,
   return !DeadBlocks.empty();
 }
 
-void llvm::FoldSingleEntryPHINodes(BasicBlock *BB,
+bool llvm::FoldSingleEntryPHINodes(BasicBlock *BB,
                                    MemoryDependenceResults *MemDep) {
-  if (!isa<PHINode>(BB->begin())) return;
+  if (!isa<PHINode>(BB->begin()))
+    return false;
 
   while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
     if (PN->getIncomingValue(0) != PN)
@@ -151,6 +152,7 @@ void llvm::FoldSingleEntryPHINodes(BasicBlock *BB,
 
     PN->eraseFromParent();
   }
+  return true;
 }
 
 bool llvm::DeleteDeadPHIs(BasicBlock *BB, const TargetLibraryInfo *TLI,