[RISCV] Enable CGP to sink splat operands of VP intrinsics

This patch brings better splat-matching to our VP support, by sinking
splat operands of VP intrinsics back into the same block as the VP
operation. The list of VP intrinsics we are interested in matches that
of the regular instructions.

Some optimization is still lacking. For instance, our VL nodes aren't
recognized as commutative, so splats must be on the RHS. Because of
this, we limit our sinking of splats to just the RHS operand for now.
Improvement in this regard can come in another patch.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D117703