[llvm] [RISCV] Use LMUL=1 for vmv_s_x_vl with non-undef passthru (PR #66659)

[Philip Reames via llvm-commits]

preames wrote:

> Have we arrived at a point where we just never reach isel with a VMV_S_X with a register class other than LMUL=1?

I think so.  Any clean up you see resulting from that?  Are there some pseudos we could delete, or are those still required by mca?

https://github.com/llvm/llvm-project/pull/66659