[llvm] [AMDGPU] Handle subregisters properly in generic operand legalizer (PR #108496)

[Matt Arsenault via llvm-commits]

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@@ -6231,10 +6231,11 @@ void SIInstrInfo::legalizeGenericOperand(MachineBasicBlock &InsertMBB,
     return;
 
   Register DstReg = MRI.createVirtualRegister(DstRC);
+  Op.setSubReg(0);
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arsenm wrote:

Instead of clearing this and using .add, could use .addReg and just not take the subreg index 

We probably don't want to be preserving the flags here. If this somehow ended up with a kill, it would be wrong 

https://github.com/llvm/llvm-project/pull/108496