[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.

After looking into this a bit I think it might be best to wait until the fast track process is at least started. There's very little details so far and there's some things that I think might end up different from how we handle extensions, e.g. should we define `__riscv_ovlt`/should it be emitted in the ELF attributes? etc.

I think it's best if we just stick to a tuning feature for now and revisit Ovlt later

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