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

[Luke Lau via llvm-commits]

lukel97 wrote:

> I am not so familiar with current LV infrastructure, how should I check the legality and cost model here?

In `VPWidenLoadEVLRecipe::computeCost` you can remove the getShuffleCost bit and use TTI.getStridedMemOpCost. 

If it's not supported then the cost should return illegal which should prevent the plan from being selected I think

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