nir: Don't skip lower_alu if only bit_count needs lowering

nir_lower_alu handles lower_bitfield_reverse, lower_bit_count, lower_mul_high, but it has an early-out that skips the iteration if none of those are needed, however the early-out check is missing one of the conditions, therefore if a driver needs only lower_bit_count, but not the other two, the bit_count lowering won't happen.