[llvm] [RISCV] Move performCombineVMergeAndVOps to RISCVVectorPeephole (PR #144076)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

LGTM in general.

I see this adds some extra `vmv`s in some cases and also removes some vtype toggles.

https://github.com/llvm/llvm-project/pull/144076