[llvm] [AMDGPU][True16][CodeGen] fix v_mov_b16_t16 index in folding pass (PR #161764)

[Brox Chen via llvm-commits]

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@@ -932,7 +931,9 @@ static MachineOperand *lookUpCopyChain(const SIInstrInfo &TII,
   for (MachineInstr *SubDef = MRI.getVRegDef(SrcReg);
        SubDef && TII.isFoldableCopy(*SubDef);
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broxigarchen wrote:

Right. I think this FoldableCopy need to updated as well. I'll create another patch for the clean up, might also replace the `if else` check with the helper

https://github.com/llvm/llvm-project/pull/161764