[RISCV] Fix bug in [l](lrint|lround) vector-cost (#151298)
Follow up on a review of bd66fd0 ([CostModel/RISCV] Fix costs of vector
[l](lrint|lround)) post-landing to fix a subtle problem with the cost
of vector [l](lrint|lround). We should use source LMUL in the case of
a narrowing op.
Co-authored-by: Luke Lau <luke@igalia.com>
