[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

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