[llvm] [CodeGen] For ad hoc aliasing, additional regUnits are needed to fix lanemask representation (PR #139206)

[Vikash Gupta via llvm-commits]

vg0204 wrote:

@jayfoad , my patch is causing some LIT test fail, so I think we should go with your fix (as it makes more sense also), but still wants to add my changes pertaining to added TableGen test. So what would be ideal way to proceed on this?

PS : Need to get it merged upstream, so can continue with #129847, (by closing #138552)  

https://github.com/llvm/llvm-project/pull/139206