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

[Luke Lau via llvm-commits]

lukel97 wrote:

I've added a new tuning flag +minimize-vl. We can probably rename it to +Ovlt if https://lists.riscv.org/g/tech-privileged/message/2487 is ratified

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