[RISCV] Fix crash when lowering fixed length insert_subvector into undef at 0 (#67535)

This fixes a crash seen in https://github.com/openxla/iree/issues/15038
and
elsewhere. We were reducing the LMUL for inserts into undef at 0 without
inserting it back into the original LMUL at the end. But we don't
actually
perform the slidedown in this path, so we can just skip reducing LMUL
here.