[RISCV] Be more aggressive about shrinking constant build_vector etype (#67175)

If LMUL is more than m1, we can be more aggressive about narrowing the
build_vector via a vsext if legal. If the narrow build_vector gets
lowered as a load, while both are linear in lmul, load uops are
generally more expensive than extend uops. If the narrow build_vector
gets lowered via dominant values, that work is linear in both #unique
elements and LMUL. So provided the number of unique values > 2, this is
a net win in work performed.