Revert "[RISCV] Shrink vslideup's LMUL when lowering fixed insert_subvector (#65997)"

This reverts commit b5ff71e2.  As described in
https://github.com/llvm/llvm-project/issues/68730, this appears to have exposed
an existing liveness issue.  Revert to green until we can figure out how to
address the root cause.

Note: This was not a clean revert.  I ended up doing it by hand.