[llvm] [RISCV] Optimize VRELOAD/VSPILL lowering if VLEN is known. (PR #74421)

[Kito Cheng via llvm-commits]

https://github.com/kito-cheng approved this pull request.

LGTM, I guess we have few rooms to optimize with the compilation time constant VLEN around the frame stuffs, and it's a good start.

https://github.com/llvm/llvm-project/pull/74421