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

[Philip Reames via llvm-commits]

preames 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.)

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