[llvm] [VPlan] Simplify reverse(reverse(x)) -> x (PR #199057)

[Mel Chen via llvm-commits]

Mel-Chen wrote:

> > I think there might be a misunderstanding here. Let me break down the workflow as I envision it:
> > ```
> > 1. A transformation to eliminate redundant `vector.reverse` operations (e.g., reverse(reverse(x)) -> x).
> > ```
> 
> This patch could serve as that starting point, yes?
> 
> The logic here LGTM.

@fhahn There seems to be a misunderstanding here. To clarify, this is the execution order of the transformations, not the landing order of the patches.

https://github.com/llvm/llvm-project/pull/199057