[llvm] [RISCV] Remove vmv.s.x and vmv.x.s lmul pseudo variants (PR #71501)

[Wang Pengcheng via llvm-commits]

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

LGTM with nits. And I need more eyes on the test diffs since they are large.

https://github.com/llvm/llvm-project/pull/71501