[llvm] [DirectX] Make dx.RawBuffer an op that can't be replaced (PR #154620)

[Justin Bogner via llvm-commits]

================
@@ -198,6 +203,111 @@ static void createLoadIntrinsic(IntrinsicInst *II, LoadInst *LI, Value *Offset,
   llvm_unreachable("Unhandled case in switch");
 }
 
+static void collectBlockUseDef(Instruction *Start,
+                               SmallVectorImpl<Instruction *> &Out) {
+  SmallPtrSet<Instruction *, 32> Visited;
+  SmallVector<Instruction *, 32> Worklist;
+  auto *BB = Start->getParent();
+
+  // Seed with direct users in this block.
+  for (User *U : Start->users()) {
+    if (auto *I = dyn_cast<Instruction>(U)) {
+      if (I->getParent() == BB)
+        Worklist.push_back(I);
+    }
+  }
+
+  // BFS over transitive users, constrained to the same block.
+  while (!Worklist.empty()) {
+    Instruction *I = Worklist.pop_back_val();
+    if (!Visited.insert(I).second)
+      continue;
+    Out.push_back(I);
+
+    for (User *U : I->users()) {
+      if (auto *J = dyn_cast<Instruction>(U)) {
+        if (J->getParent() == BB)
+          Worklist.push_back(J);
+      }
+    }
+    for (Use &V : I->operands()) {
+      if (auto *J = dyn_cast<Instruction>(V)) {
+        if (J->getParent() == BB && V != Start)
+          Worklist.push_back(J);
+      }
+    }
+  }
+
+  // Order results in program order.
+  DenseMap<const Instruction *, unsigned> Ord;
+  unsigned Idx = 0;
+  for (Instruction &I : *BB)
+    Ord[&I] = Idx++;
+
+  llvm::sort(Out, [&](Instruction *A, Instruction *B) {
+    return Ord.lookup(A) < Ord.lookup(B);
+  });
+}
+
+static void phiNodeRemapHelper(PHINode *Phi, BasicBlock *BB,
+                               IRBuilder<> &Builder,
+                               SmallVector<Instruction *> &UsesInBlock) {
+
+  ValueToValueMapTy VMap;
+  Value *Val = Phi->getIncomingValueForBlock(BB);
+  VMap[Phi] = Val;
+  Builder.SetInsertPoint(&BB->back());
+  for (Instruction *I : UsesInBlock) {
+    // don't clone over the Phi just remap them
+    if (auto *PhiNested = dyn_cast<PHINode>(I)) {
+      VMap[PhiNested] = PhiNested->getIncomingValueForBlock(BB);
+      continue;
+    }
+    Instruction *Clone = I->clone();
+    RemapInstruction(Clone, VMap,
+                     RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
+    Builder.Insert(Clone);
+    VMap[I] = Clone;
+  }
+}
+
+static void phiNodeReplacement(IntrinsicInst *II,
+                               SmallVector<Instruction *> &PrevBBDeadInsts,
+                               SetVector<BasicBlock *> &DeadBB) {
+  SmallVector<Instruction *> CurrBBDeadInsts;
+  for (User *U : II->users()) {
+    auto *Phi = dyn_cast<PHINode>(U);
+    if (!Phi)
+      continue;
+
+    IRBuilder<> Builder(Phi);
+    SmallVector<Instruction *> UsesInBlock;
+    collectBlockUseDef(Phi, UsesInBlock);
+    bool HasReturnUse = isa<ReturnInst>(UsesInBlock[UsesInBlock.size() - 1]);
----------------
bogner wrote:

Isn't this just `back()`?
```suggestion
    bool HasReturnUse = isa<ReturnInst>(UsesInBlock.back());
```

https://github.com/llvm/llvm-project/pull/154620