[llvm] [VPlan] Extract reverse operation for reverse accesses (PR #146525)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

+1 on splitting this out, I think this works well with the direction of splitting up big recipes into smaller ones. Just an idea about possibly inserting the reverses in `tryToWiden` but otherwise generally LGTM

https://github.com/llvm/llvm-project/pull/146525