[llvm] [LoopVectorize] Generate wide active lane masks (PR #147535)

[David Sherwood via llvm-commits]

https://github.com/david-arm approved this pull request.

LGTM with nit addressed! I think we can probably clean up the dead active lane masks in a follow-up patch, since the new flag is off by default anyway.

https://github.com/llvm/llvm-project/pull/147535