[llvm] [RISCV] Move RISCVDeadRegisterDefinitions to post vector regalloc (PR #90636)

[Luke Lau via llvm-commits]

lukel97 wrote:

I've updated this to run after vector regalloc but before scalar regalloc, so we should have the best of both worlds now. 

https://github.com/llvm/llvm-project/pull/90636