[llvm] [WIP][CodeGen] Encode liveness for COPY instructions after virtRegRewriter pass. (PR #151123)

[Matt Arsenault via llvm-commits]

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@@ -495,7 +568,11 @@ void VirtRegRewriter::handleIdentityCopy(MachineInstr &MI) {
   // give us additional liveness information: The target (super-)register
   // must not be valid before this point. Replace the COPY with a KILL
   // instruction to maintain this information.
-  if (MI.getOperand(1).isUndef() || MI.getNumOperands() > 2) {
+
+  // Avoid COPY with an exact 3 operand, wiith third operand be Mask, as
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arsenm wrote:

```suggestion
  // Avoid COPY with an exact 3 operand, with third operand be Mask, as
```

https://github.com/llvm/llvm-project/pull/151123