Add cost model for broadcast shuffle in RISCVTTIImpl::getShuffleCost
with scalable vector. The cost model might not the best.
For scalable vector, BasicTTIImpl::getShuffleCost return invalid cost,
so this patch relies on the existing cost model in BasicTTIImpl.
I am curious with this one. I would expect this case to be 1 too. How is different to the other ones?