[llvm] [VPlan] Always consider register pressure on RISC-V (PR #156951)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Overall seems fine, but I'd prefer to avoid adding super specialized TTI hooks if possible. That being said, I am not sure if there's a good alternative for this case

Agreed, ideally I would have thought that every target would want to opt into pruning VFs based on reg pressure but given the MVE regression in https://github.com/llvm/llvm-project/pull/141736 and updates in https://github.com/llvm/llvm-project/pull/149056 it seems to be a very different story on AArch64. I didn't want to disturb any other targets but can't really think of an existing hook we could reuse.

https://github.com/llvm/llvm-project/pull/156951