[llvm] [DAGCombiner] Move handling of atomic loads from SystemZ to DAGCombiner (NFC). (PR #86484)

[Matt Arsenault via llvm-commits]

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@@ -1454,6 +1454,28 @@ class TargetLoweringBase {
            getLoadExtAction(ExtType, ValVT, MemVT) == Custom;
   }
 
+  /// Same as getLoadExtAction, but for atomic loads.
+  LegalizeAction getAtomicLoadExtAction(unsigned ExtType, EVT ValVT,
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arsenm wrote:

The usage of getAtomicLoadExtAction I'm more concerned with is the semantic load bearing pieces like [this](https://github.com/llvm/llvm-project/blob/4ddd4ed7fe15a356dace649e18492dd01071f475/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp#L4072), adding hidden properties to these atomics. It would be better if we just moved the atomic-ness into the MMOs to avoid this magic bit 

https://github.com/llvm/llvm-project/pull/86484