[RISCV] Match vwmulsu_vx with scalar splat input.

This is a more generic version of D119110 that uses MaskedValueIsZero
to do the matching and SimplifyDemandedBits to remove any unneeded
AND instructions.

Tests were taken from D119110.

Reviewed By: Chenbing.Zheng

Differential Revision: https://reviews.llvm.org/D119622