[llvm] Address Codegen bug related to marking subregister MachineOperand defines as undef (PR #134929)

[Matt Arsenault via llvm-commits]

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@@ -1570,6 +1570,29 @@ void LiveIntervals::handleMove(MachineInstr &MI, bool UpdateFlags) {
 
   HMEditor HME(*this, *MRI, *TRI, OldIndex, NewIndex, UpdateFlags);
   HME.updateAllRanges(&MI);
+
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arsenm wrote:

I'm not 100% sure if handleMove should be responsible for updating the actual MI. updateAllRanges conservatively strips kill flags (which is kind of annoying), but seems to not touch the MIR 

https://github.com/llvm/llvm-project/pull/134929