[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
> 
> 
> 
> Why don't we just support it at the beginning? Those are just some optimization feature markers.

I'm happy to do that, just wasn't sure if people wanted to wait until it was ratified or not. I can open up a separate PR, since we'll need to handle it in the ISA parser etc. 

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