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

We currently shrink the type of vmv_s_x_vl to LMUL=1 when its passthru
is
undef to avoid constraining the register allocator since it ignores
LMUL.
This patch relaxes it for non-undef passthrus, which occurs when
lowering
insert_vector_elt.