[llvm] TwoAddressInstruction: Fix assert on undef operand with -early-live-intervals (PR #125518)

[Matt Arsenault via llvm-commits]

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@@ -489,6 +489,9 @@ MachineInstr *TwoAddressInstructionImpl::findOnlyInterestingUse(
     bool &IsDstPhys) const {
   MachineOperand *UseOp = nullptr;
   for (MachineOperand &MO : MRI->use_nodbg_operands(Reg)) {
+    if (MO.isUndef())
+      continue;
+
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arsenm wrote:

It probably should, but I'd need to look exactly at all the implementations of isPlainlyKilled. At least the LiveIntervals one checks there's only one value in the interval 

https://github.com/llvm/llvm-project/pull/125518