[RISCV] Mark vsadd(u)_vl as commutable

This allows fixed length vectors involving splats on the LHS to commute into the _vx form of the instruction. Oddly, the generic canonicalization rules appear to catch the scalable vector cases. I haven't fully dug in to understand why, but I suspect it's because of a difference in how we represent splats (splat_vector vs build_vector).

Differential Revision: https://reviews.llvm.org/D129302