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

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> > There is another potential issue. Some targets might not support strided operations, so would be good to add a check and add new flag(possibly?) in load/stores recipes for strided ops
> 
> I don't think I'm too concerned about other targets since as far as I'm aware RISC-V is the only target that can lower llvm.experimental.vp.reverse today. 

Today - maybe. But EVL vectorizer is not RISCV only (generally speaking!), so need to provide full correctness here for all potential targets.

> And we don't seem to currently check if it's supported anyway?

Yes, because we never emit strided ops in LV for now. To emit it, need to correctly implement legality checks and cost model

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