[RISCV] Add cost modelling for vector widenning reduction.

In RVV, we use vwredsum.vs and vwredsumu.vs for vecreduce.add(ext(Ty A)) if the result type's width is twice of the input vector's SEW-width. In this situation, the cost of extended add reduction should be same as single-width add reduction. So as the vector float widenning reduction.

Differential Revision: https://reviews.llvm.org/D129994