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

[Matt Arsenault via llvm-commits]

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@@ -1166,9 +1166,18 @@ void AMDGPUPromoteAllocaImpl::promoteAllocaToVector(AllocaAnalysis &AA) {
   });
 
   // Now fixup the placeholders.
-  for (Instruction *Placeholder : Placeholders) {
-    Placeholder->replaceAllUsesWith(
-        Updater.GetValueInMiddleOfBlock(Placeholder->getParent()));
+  SmallVector<WeakVH> PlaceholderToNewVal(Placeholders.size());
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arsenm wrote:

This usage of WeakVH doesn't make sense. I meant use WeakVH in an existing structure. This usage doesn't check if the value is null / was deleted before using it later 

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