[llvm] [RISCV] Support postRA vsetvl insertion pass (PR #70549)

[Piyou Chen via llvm-commits]

BeMg wrote:

@lukel97 Thanks for collaborating on these patches. And the reviewer's comments also made this patch possible.

> Nice, great work :)
> 
> Now that rematerialization and partial spilling are unblocked, were you planning to take a look at either of them? I have half a proof-of-concept patch lying around for rematerialization that I could pick up again, but only if you haven't already started that work.

Feel free to pick up on rematerialization. I haven't started on it yet. :)

I will explore whether we could do something on scheduler to control vsetvl insertion.

https://github.com/llvm/llvm-project/pull/70549