[llvm] [AMDGPU] Remove unsound idempotent atomicrmw to atomic load lowering (PR #218377)

[Matt Arsenault via llvm-commits]

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@@ -613,9 +613,6 @@ class SITargetLowering final : public AMDGPUTargetLowering {
   void emitExpandAtomicLoad(LoadInst *LI) const override;
   void emitExpandAtomicStore(StoreInst *SI) const override;
 
-  LoadInst *
-  lowerIdempotentRMWIntoFencedLoad(AtomicRMWInst *AI) const override;
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arsenm wrote:

Why does the hook still exist then? There also wasn't anything target specific in here 

https://github.com/llvm/llvm-project/pull/218377