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

[Matt Arsenault via llvm-commits]

================
@@ -451,13 +497,33 @@ bool SDWASrcOperand::convertToSDWA(MachineInstr &MI, const SIInstrInfo *TII) {
   }
   copyRegOperand(*Src, *getTargetOperand());
   if (!IsPreserveSrc) {
-    SrcSel->setImm(getSrcSel());
+    SdwaSel ExistingSel = static_cast<SdwaSel>(SrcSel->getImm());
+    SrcSel->setImm(combineSdwaSel(ExistingSel, getSrcSel()).value());
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arsenm wrote:

```suggestion
    SrcSel->setImm(*combineSdwaSel(ExistingSel, getSrcSel()));
```

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