[RISCV] Match VF variants for masked VFRDIV/VFRSUB

This patch follows up on D117697 to help the simple binary operations
behave similarly in the presence of masks.

It also enables CGP sinking support for vp.fdiv and vp.fsub intrinsics,
now that VFRDIV and VFRSUB are consistently matched with a LHS splat for
masked and unmasked variants.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D117783