[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.
+  DenseMap<Value *, Value *> PlaceholderMap;
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arsenm wrote:

Should this be using WeakVH instead 

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