[llvm] [RISCV] Move RISCVInsertVSETVLI to after phi elimination (PR #91440)

[Luke Lau via llvm-commits]

lukel97 wrote:

I've separated out the changes around vxrm/csr into #91701 and landed them. This PR is now rebased on top so we should be left with the interesting test diffs now

https://github.com/llvm/llvm-project/pull/91440