[llvm] [TwoAddressInstruction] Propagate undef flags for partial defs (PR #79286)

[Matt Arsenault via llvm-commits]

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@@ -1991,11 +1997,25 @@ eliminateRegSequence(MachineBasicBlock::iterator &MBBI) {
       MI.removeOperand(j);
   } else {
     if (LIS) {
-      // Force interval recomputation if we moved from full definition
-      // of register to partial.
-      if (DefIsPartial && LIS->hasInterval(DstReg) &&
-          MRI->shouldTrackSubRegLiveness(DstReg))
+      // Force live interval recomputation if we moved to a partial defintion
----------------
arsenm wrote:

```suggestion
      // Force live interval recomputation if we moved to a partial definition
```

https://github.com/llvm/llvm-project/pull/79286