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

[Vikash Gupta via llvm-commits]

vg0204 wrote:

Added the RFC : https://discourse.llvm.org/t/llvm-codegen-rfc-add-mo-lanemask-type-and-a-new-copy-lanemask-instruction/88021

Gentle ping : @arsenm , @jayfoad , @cdevadas , @ssahasra , @rovka , @qcolombet 

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