[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

@@ -1149,14 +1149,16 @@ 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()) {
+    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) {
                 call->replaceAllUsesWith(tag);
                 // Push to the removed instructions to trigger `finalize` to
                 // return the correct result.
                 // Also so that we don't have to worry about iterator invalidation...
+                use = UndefValue::get(use->getType());
                 removed.push_back(call);
             }
         }