[MTE] Remove redundant helper function.

[fmayer]

Summary

Looking at PostDominatorTree::dominates, we can see that has the same
logic (with the addition of handling Phi nodes - which are not used as inputs in
this pass) as the helper function.

[eugenis]

Ah interesting, PostDominatorTree::dominates has been added just a few months after our helper function: https://reviews.llvm.org/D68469 vs https://reviews.llvm.org/D70049.

LGTM

llvm/lib/Target/AArch64/AArch64StackTagging.cpp

void AArch64StackTagging::alignAndPadAlloca(AllocaInfo &Info) {
   NewAI->copyMetadata(*Info.AI);
 
   auto *NewPtr = new BitCastInst(NewAI, Info.AI->getType(), "", Info.AI);
   Info.AI->replaceAllUsesWith(NewPtr);
   Info.AI->eraseFromParent();
   Info.AI = NewAI;
 }
 
-// Helper function to check for post-dominance.
-static bool postDominates(const PostDominatorTree *PDT, const IntrinsicInst *A,
-                          const IntrinsicInst *B) {
-  const BasicBlock *ABB = A->getParent();
-  const BasicBlock *BBB = B->getParent();
-
-  if (ABB != BBB)
-    return PDT->dominates(ABB, BBB);
-
-  for (const Instruction &I : *ABB) {
-    if (&I == B)
-      return true;
-    if (&I == A)
-      return false;
-  }
-  llvm_unreachable("Corrupt instruction list");
-}
-
 // FIXME: check for MTE extension
 bool AArch64StackTagging::runOnFunction(Function &Fn) {
   if (!Fn.hasFnAttribute(Attribute::SanitizeMemTag))
     return false;
 
   if (UseStackSafety)
     SSI = &getAnalysis<StackSafetyGlobalInfoWrapperPass>().getResult();
   F = &Fn;
if (UnrecognizedLifetimes.empty() && Info.LifetimeStart.size() == 1 &&
       IntrinsicInst *Start = Info.LifetimeStart[0];
       IntrinsicInst *End = Info.LifetimeEnd[0];
       uint64_t Size =
           cast<ConstantInt>(Start->getArgOperand(0))->getZExtValue();
       Size = alignTo(Size, kTagGranuleSize);
       tagAlloca(AI, Start->getNextNode(), Start->getArgOperand(1), Size);
       // We need to ensure that if we tag some object, we certainly untag it
       // before the function exits.
-      if (PDT != nullptr && postDominates(PDT, End, Start)) {
+      if (PDT != nullptr && PDT->dominates(End, Start)) {
         untagAlloca(AI, End, Size);
       } else {
         SmallVector<Instruction *, 8> ReachableRetVec;
         unsigned NumCoveredExits = 0;
         for (auto &RI : RetVec) {
           if (!isPotentiallyReachable(Start, RI, nullptr, DT))
             continue;
           ReachableRetVec.push_back(RI);