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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I've been thinking of introducing a COPY for a lanemask as a substitute for the copy bundles splitkit produces, but it should be a separate opcode instead of acting like an implicit operand on COPY.

I think you should introduce the new operand type first, then the opcode separately from touching VirtRegMap 

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