[llvm] [LV] Use wide lane masks as the canonical form when tail-folding & interleaving (PR #209484)

[Benjamin Maxwell via llvm-commits]

https://github.com/MacDue commented:

This generally LGTM, just a few small comments. I'll defer to Paul for the naming. 

These VPInstructions would also be useful for my later work too, thanks 👍 

https://github.com/llvm/llvm-project/pull/209484