[llvm] [AArch64][ISel] Add custom lowering for clmul nxv8i16 (PR #195893)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm commented:

In general this look good to me.

It's up to you but if you want to improve the generated code further I'll note that only half the `pmul` vectors have useful data so rather than extracting the high half of each operand I think you can just byte swap one of them and do a single `pmul`. At that point I think you are just a combination of eorbt and eortb instructions away from the final result.

https://github.com/llvm/llvm-project/pull/195893