[llvm] [AMDGPU] Fix a potential use-after-erase in `AMDGPUPromoteAlloca` pass (PR #174529)

[Shilei Tian via llvm-commits]

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@@ -1166,9 +1166,18 @@ void AMDGPUPromoteAllocaImpl::promoteAllocaToVector(AllocaAnalysis &AA) {
   });
 
   // Now fixup the placeholders.
+  DenseMap<Value *, Value *> PlaceholderMap;
   for (Instruction *Placeholder : Placeholders) {
-    Placeholder->replaceAllUsesWith(
-        Updater.GetValueInMiddleOfBlock(Placeholder->getParent()));
+    Value *NewVal = Updater.GetValueInMiddleOfBlock(Placeholder->getParent());
+    Placeholder->replaceAllUsesWith(NewVal);
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shiltian wrote:

The best approach would be something like having `replaceAllUsesWith` "notify" `SSAUpdater` in some way, but that is far beyond the scope here, and I expect it would take a lot of work.

https://github.com/llvm/llvm-project/pull/174529