[llvm] [RISCV] Convert AVLs with vlenb to VLMAX where possible (PR #97800)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> > I think we can rename `RISCVFoldMasks` to `PostISelRVVOptimization` or whatever.
> 
> Maybe something like `RISCVVectorPeephole`?

Make sense to me.

https://github.com/llvm/llvm-project/pull/97800