[llvm] [AMDGPU] misched: avoid subregister dependencies (PR #140255)

[Matt Arsenault via llvm-commits]

================
@@ -627,6 +627,124 @@ GCNSubtarget::getMaxNumVectorRegs(const Function &F) const {
   return std::pair(MaxNumVGPRs, MaxNumAGPRs);
 }
 
+// Check to which source operand UseOpIdx points to and return a pointer to the
+// operand of the corresponding source modifier.
+// Return nullptr if UseOpIdx either doesn't point to src0/1/2 or if there is no
+// operand for the corresponding source modifier.
+static const MachineOperand *
+getVOP3PSourceModifierFromOpIdx(const MachineInstr *UseI, int UseOpIdx,
+                                const SIInstrInfo &InstrInfo) {
+  AMDGPU::OpName UseModName;
+  AMDGPU::OpName UseName =
+      AMDGPU::getOperandIdxName(UseI->getOpcode(), UseOpIdx);
+  switch (UseName) {
+  case AMDGPU::OpName::src0:
+    UseModName = AMDGPU::OpName::src0_modifiers;
+    break;
----------------
arsenm wrote:

```suggestion
    return InstrInfo.getNamedOperand(*UseI, AMDGPU::OpName::src0_modifiers)
```

https://github.com/llvm/llvm-project/pull/140255