[llvm] [RISCV] Separate doLocalPostpass into new pass and move to post vector regalloc (PR #88295)

[Luke Lau via llvm-commits]

lukel97 wrote:

> After this patch, additional `vmv.v.v` instructions will be generated:

This is to be expected as we now have some extra vsetvlis that live through vector regalloc (but are then coalesced before scalar regalloc), so the live ranges may be slightly longer. For what it's worth I don't believe this is a serious regression, there's some other places where we remove copies as well.

Scheduling and vector regalloc should significantly improve once #70549 lands

https://github.com/llvm/llvm-project/pull/88295