[RISCV] Add cost model for SK_Broadcast

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.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D124101