[llvm] [AMDGPU] Account for existing SDWA selections (PR #123221)

[Jeffrey Byrnes via llvm-commits]

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@@ -108,6 +111,42 @@ class SDWAOperand {
   MachineOperand *getReplacedOperand() const { return Replaced; }
   MachineInstr *getParentInst() const { return Target->getParent(); }
 
+  /// Combine an SDWA instruction's existing SDWA selection \p
+  /// ExistingSel with the SDWA selection \p OpSel of its operand. If
+  /// the selections are compatible, return the combined selection,
+  /// otherwise return a nullopt. For example, if we have ExistingSel
+  /// = BYTE_0 Sel and FoldedSel WORD_1 Sel:
+  ///     BYTE_0 Sel (WORD_1 Sel (%X)) -> BYTE_2 Sel (%X)
+  std::optional<SdwaSel> combineSdwaSel(SdwaSel ExistingSel,
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jrbyrnes wrote:

Can we disallow the bad combos when checking if its possible to convert the op / MI ? This allows for early exit + removal of the std::optional

https://github.com/llvm/llvm-project/pull/123221