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

[Christudasan Devadasan via llvm-commits]

https://github.com/cdevadas commented:

Overall, the patch looks good to me. See my comments for some fixups and nits.
How this new MO_Lanemask operand and the newly introduced opcodes involving it will interact with various CodeGen passes is yet to be observed. Looking forward to that. Especially how we can replace the CopyBundles with the help fo the LaneMask MO field.

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