[llvm] [RISCV] Avoid vl toggles when lowering vector_splice/experimental_vp_splice and add +minimize-vl tuning feature (PR #146746)

[Min-Yih Hsu via llvm-commits]

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

LGTM. 
The tuning flag will be pretty useful, thanks

https://github.com/llvm/llvm-project/pull/146746