[llvm] [AMDGPU] si-peephole-sdwa: Disable V_CNDMASK_B32 conversion with sext (PR #140760)

[Matt Arsenault via llvm-commits]

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@@ -430,6 +430,13 @@ bool SDWASrcOperand::convertToSDWA(MachineInstr &MI, const SIInstrInfo *TII) {
   case AMDGPU::V_CVT_PK_F32_BF8_sdwa:
     // Does not support input modifiers: noabs, noneg, nosext.
     return false;
+  case AMDGPU::V_CNDMASK_B32_sdwa:
+    // FIXME SISrcMods uses the same bitmask for SEXT and NEG
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arsenm wrote:

```suggestion
    // FIXME: SISrcMods uses the same bitmask for SEXT and NEG
```

Is it only this one bit? Also make this clearer that both bits should work, we just didn't define the enums appropriately 

https://github.com/llvm/llvm-project/pull/140760