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

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

Is this still profitable even for cases when there's no shift needed, e.g. would a vsetvli be better than a single csrr?

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