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

[Mel Chen via llvm-commits]

Mel-Chen wrote:

https://github.com/llvm/llvm-project/pull/155579
Since separating the reverse mask also involves cost model and requires morel implementation during EVL lowering, I decided to split these two changes to reduce the complexity of this PR.

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