[RISCV] Add isel patterns for masked RISCVISD::FMA_VL with RISCVISD::FNEG_VL.

This helps us form vfnmsub, vfnmadd, and vfmusb from masked VP
intrinsics.

I've used "srcvalue" for the mask parameter in the fneg nodes. We
can't match "V0" because that doesn't ensure the mask the is the same.
Instead it matches two different nodes and generates two copies to
V0 of those separate values.

Reviewed By: rogfer01

Differential Revision: https://reviews.llvm.org/D120287