[llvm] [LV][EVL] Generate negative strided load/store for reversed load/store (PR #123608)

[Mel Chen via llvm-commits]

Mel-Chen wrote:

> (Only skimming the thread)
> 
> If we decide to move forward with this, we'll need someway to handle a reversed stride store of a binop of two reversed loads. As of my recent instcombine change to support vp.reverse, we can elide the reverses entirely. (Basically, our baseline improved, so we need to make sure this patch improves on the new baseline.)

I remember @arcbbb have a patch for reverse pair accesses in our downstream. 

https://github.com/llvm/llvm-project/pull/123608