[SSAUpdater] Only iterate blocks modified by CheckIfPHIMatches() in RecordMatchingPHIs() (#153596)

In #100281, we use
`TaggedBlocks` to record blocks modified by `CheckIfPHIMatche()`, so do
not need to clear every block in `BlockList` if `CheckIfPHIMatches()`
match failed.

If `CheckIfPHIMatches()` match succeed, we can reuse `TaggedBlocks` to
only record matching PHIs for modified blocks, avoid checking every
block in `BlockList` to see if `PHITag` is set.

Author: Enna1

llvm/include/llvm/Transforms/Utils/SSAUpdaterImpl.h

@@ -366,7 +366,7 @@ class SSAUpdaterImpl {
         continue;
 
       // Look for an existing PHI.
-      FindExistingPHI(Info->BB, BlockList);
+      FindExistingPHI(Info->BB);
       if (Info->AvailableVal)
         continue;
 
@@ -412,19 +412,19 @@ class SSAUpdaterImpl {
 
   /// FindExistingPHI - Look through the PHI nodes in a block to see if any of
   /// them match what is needed.
-  void FindExistingPHI(BlkT *BB, BlockListTy *BlockList) {
+  void FindExistingPHI(BlkT *BB) {
     SmallVector<BBInfo *, 20> TaggedBlocks;
     for (auto &SomePHI : BB->phis()) {
       if (CheckIfPHIMatches(&SomePHI, TaggedBlocks)) {
-        RecordMatchingPHIs(BlockList);
+        RecordMatchingPHIs(TaggedBlocks);
         break;
       }
     }
   }
 
   /// CheckIfPHIMatches - Check if a PHI node matches the placement and values
   /// in the BBMap.
-  bool CheckIfPHIMatches(PhiT *PHI, SmallVectorImpl<BBInfo *> &TaggedBlocks) {
+  bool CheckIfPHIMatches(PhiT *PHI, BlockListTy &TaggedBlocks) {
     // Match failed: clear all the PHITag values. Only need to clear visited
     // blocks.
     auto Cleanup = make_scope_exit([&]() {
@@ -484,15 +484,15 @@ class SSAUpdaterImpl {
 
   /// RecordMatchingPHIs - For each PHI node that matches, record it in both
   /// the BBMap and the AvailableVals mapping.
-  void RecordMatchingPHIs(BlockListTy *BlockList) {
-    for (typename BlockListTy::iterator I = BlockList->begin(),
-           E = BlockList->end(); I != E; ++I)
-      if (PhiT *PHI = (*I)->PHITag) {
-        BlkT *BB = PHI->getParent();
-        ValT PHIVal = Traits::GetPHIValue(PHI);
-        (*AvailableVals)[BB] = PHIVal;
-        BBMap[BB]->AvailableVal = PHIVal;
-      }
+  void RecordMatchingPHIs(BlockListTy &TaggedBlocks) {
+    for (BBInfo *Block : TaggedBlocks) {
+      PhiT *PHI = Block->PHITag;
+      assert(PHI && "PHITag didn't set?");
+      BlkT *BB = PHI->getParent();
+      ValT PHIVal = Traits::GetPHIValue(PHI);
+      (*AvailableVals)[BB] = PHIVal;
+      BBMap[BB]->AvailableVal = PHIVal;
+    }
   }
 };