[llvm] [IR][AtomicExpand] Add elementwise modifier to atomicrmw; automatically expand for all targets (PR #189517)

[Antonio Frighetto via llvm-commits]

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@@ -920,7 +927,7 @@ class AtomicRMWInst : public Instruction {
 
 private:
   void Init(BinOp Operation, Value *Ptr, Value *Val, Align Align,
-            AtomicOrdering Ordering, SyncScope::ID SSID);
+            AtomicOrdering Ordering, SyncScope::ID SSID, bool Elementwise);
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antoniofrighetto wrote:

Just wondering, any reason why favored encoding elementwise as a separate bool and not part of `SubclassData`, like volatile and friends? Should be still left one spare bit to use?

https://github.com/llvm/llvm-project/pull/189517