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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> NOTE: Currently, as we do not support any MachineOperand or MachineOperand attribute( associated with Register operand) that could hold liveness information as the 64-bit laneBitmask,

It would be trivial to add an MO_LaneMask

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