[llvm] [LV][EVL]Support reversed loads/stores. (PR #88025)

[Florian Hahn via llvm-commits]

https://github.com/fhahn approved this pull request.

LGTM, but please move the code to create vp.reverse with all-true mask to a static helper to reduce duplication and make sure that it is done consistently.

https://github.com/llvm/llvm-project/pull/88025