[llvm] [CodeGen] Add MO_LaneMask type and a new COPY_LANEMASK instruction (PR #151944)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

I think this is basically OK, it just needs some attention to detail and some improvement of the description and documentation of the new operand type and instruction.

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