[RISCV][ISel] Remove the commutative flag on SUB

I wasn't able to produce a testcase for that because right now VWSUB is
only generated from VWSUB_W and from there to trigger the commutative
bug we would need to grab VWSUB where the splat value is on the LHS,
which is currently not matched.

Differential Revision: https://reviews.llvm.org/D134701