[llvm] [InstCombine] Iterative replacement in PtrReplacer (PR #137215)

[Matt Arsenault via llvm-commits]

================
@@ -270,80 +268,132 @@ class PointerReplacer {
 } // end anonymous namespace
 
 bool PointerReplacer::collectUsers() {
-  if (!collectUsersRecursive(Root))
-    return false;
-
-  // Ensure that all outstanding (indirect) users of I
-  // are inserted into the Worklist. Return false
-  // otherwise.
-  return llvm::set_is_subset(ValuesToRevisit, Worklist);
-}
+  SmallVector<Instruction *> Worklist;
+  SmallSetVector<Instruction *, 4> ValuesToRevisit;
+
+  auto PushUsersToWorklist = [&](Instruction *Inst) {
+    for (auto *U : Inst->users()) {
+      if (auto *I = dyn_cast<Instruction>(U)) {
+        if (!isAvailable(I) && !ValuesToRevisit.contains(I))
+          Worklist.emplace_back(I);
+      }
+    }
+  };
 
-bool PointerReplacer::collectUsersRecursive(Instruction &I) {
-  for (auto *U : I.users()) {
-    auto *Inst = cast<Instruction>(&*U);
+  PushUsersToWorklist(&Root);
+  while (!Worklist.empty()) {
+    auto *Inst = Worklist.pop_back_val();
----------------
arsenm wrote:

```suggestion
    Instruction *Inst = Worklist.pop_back_val();
```

https://github.com/llvm/llvm-project/pull/137215