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

[Matt Arsenault via llvm-commits]

================
@@ -474,6 +476,77 @@ bool VirtRegRewriter::readsUndefSubreg(const MachineOperand &MO) const {
   return true;
 }
 
+// Return LaneBitmask value as unint64_t for PhysReg assigned to MO,
----------------
arsenm wrote:

```suggestion
// Return LaneBitmask value as uint64_t for PhysReg assigned to MO,
```

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