[llvm] [RISCV] Use vsetvli instead of vlenb in Prologue/Epilogue (PR #113756)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM w/minor suggestion.

As a follow up, I think we should consider a) enabling this by default given a large fraction of existing hardware benefits and we don't have anyone raising concerns about regressions on the rest and b) extending this for non-power-of-twos by using mulimm on the resulting value.  

https://github.com/llvm/llvm-project/pull/113756