[llvm] [CodeGen] Add laneBitmask as new MachineOperand type, utilised by newly defined COPY_LANEMASK instruction (PR #151944)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Please post an RFC on discourse explaining what problem this solves and how.

My immediate desire for this is to stop using the poorly-defined COPY bundles that split kit uses if only some lanes are live. It isn't a standard bundle format with a leading BUNDLE header instruction, and also requires snippet copy identification to do more work to identify these bundles 

https://github.com/llvm/llvm-project/pull/151944