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

[Luke Lau via llvm-commits]

lukel97 wrote:

Reverted in fc13353e10b443101b7304c5f90d70a06e20e589 and reapplied in af82d01fbbce7808605f3a8b22dd1ca7fdec7886. The original commit was calling `shrinkToUse` on an interval whose def was erased, the reapplied version fixes it by calling `shrinkToUse` before we erase it and removing the interval afterwards if needed.

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