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

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM

For anyone reading along, the interesting part of the test diff no longer exists in this review.  It was handled by the dependent change which already landed, and was discussed in detail there.  What's left is straight forward with no surprising test changes.  

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