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

[Craig Topper via llvm-commits]

topperc 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?

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