[AllocOpt] fix iterator invalidation

We might previously accidentally visit this use after deletion, if the
orig_inst ended up back in the workqueue.

Fixes #41916

src/llvm-alloc-opt.cpp

@@ -1149,7 +1149,9 @@ void Optimizer::optimizeTag(CallInst *orig_inst)
 {
     auto tag = orig_inst->getArgOperand(2);
     // `julia.typeof` is only legal on the original pointer, no need to scan recursively
-    for (auto user: orig_inst->users()) {
+    size_t last_deleted = removed.size();
+    for (auto &use: orig_inst->uses()) {
+        auto user = use.getUser();
         if (auto call = dyn_cast<CallInst>(user)) {
             auto callee = call->getCalledOperand();
             if (pass.typeof_func == callee) {
@@ -1161,6 +1163,8 @@ void Optimizer::optimizeTag(CallInst *orig_inst)
             }
         }
     }
+    while (last_deleted < removed.size())
+        removed[last_deleted++]->replaceUsesOfWith(orig_inst, UndefValue::get(orig_inst->getType()));
 }
 
 void Optimizer::splitOnStack(CallInst *orig_inst)