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

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

Small style suggestion ... try rewriting the commit description without saying "this patch"?

The first line of the commit description is unnecessarily long. I believe it could be shortened to the following with negligible loss of information:

> [CodeGen] Add MO_LaneMask type and a new COPY_LANEMASK instruction

Also, as an informed layman (but not familiar with backend jargon), this sentence didn't really tell me much: 

> This patch adds a new MachineOperand type to represent the laneBitmask as MO_LaneMask that can be used in the instructions to **represent the relevant information associated with the register** operands of the same such as liveness. 


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