[llvm] [AMDGPU] Fix SDWA selection combining (PR #218512)

[Arseniy Obolenskiy via llvm-commits]

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@@ -309,24 +309,27 @@ static MachineOperand *findSingleRegDef(const MachineOperand *Reg,
   return MRI->getOneDef(Reg->getReg());
 }
 
-/// Combine an SDWA instruction's existing SDWA selection \p Sel with
+/// Combine an SDWA instruction's existing source selection \p Sel with
 /// the SDWA selection \p OperandSel of its operand. If the selections
 /// are compatible, return the combined selection, otherwise return a
-/// nullopt.
+/// nullopt. Destination selections are never composed, see
+/// SDWADstOperand::canCombineSelections.
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aobolensk wrote:

added more details in the comment

https://github.com/llvm/llvm-project/pull/218512