[RISCV] Reduce LMUL when index is known when lowering insert_vector_elt (#66087)

Continuing on from #65997, if the index of insert_vector_elt is a
constant then we can work out what the minimum number of registers will
be needed for the slideup and choose a smaller type to operate on.

This reduces the LMUL for not just the slideup but also for the scalar
insert.