[llvm] [GlobalISel][AMDGPU][AArch64] Fix GlobalISel copy propagation (PR #188781)

[Matt Arsenault via llvm-commits]

================
@@ -1048,8 +1048,15 @@ static bool selectCopy(MachineInstr &I, const TargetInstrInfo &TII,
 
     const TypeSize SrcSize = TRI.getRegSizeInBits(*SrcRC);
     const TypeSize DstSize = TRI.getRegSizeInBits(*DstRC);
+    unsigned SrcSubReg = I.getOperand(1).getSubReg();
     unsigned SubReg;
 
+    if (SrcSubReg) {
+      if (!RBI.constrainGenericRegister(DstReg, *DstRC, MRI))
+        return false;
+      return true;
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arsenm wrote:

```suggestion
      return RBI.constrainGenericRegister(DstReg, *DstRC, MRI);
```

https://github.com/llvm/llvm-project/pull/188781