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

[Ryan Buchner 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);
+
+  // Check to make sure that there are no subreg defintions marked undef
+  // after the moved operation. If so, mark the current instruction as undef
+  // instead.
+  for (MachineOperand &MO : MI.operands()) {
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bababuck wrote:

I did do some more digging and was able to better utilize the `MachineInst` functions to clean up some of the code. 

I think that expressing this in terms of `operand-is-not-undef-but-not-live-at-this-point` does make sense because that is more similar to how the verifier works.

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