[RISCV][VP] support vp.reduce.mul by ExpandVectorPredication

Most of VP intrinsics are implemented in RISC-V backends, but
vp.reduce.mul (element length > 1) does not yet. Legalizes vp.reduce.mul
using ExpandVectorPredication Pass.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D139721