[RISCV] Add RISCVISD nodes for vfwadd/vfwsub.

Add a DAG combine to form these from FADD_VL/FSUB_VL and FP_EXTEND_VL.

This makes it similar to other widening ops and allows us to handle
using the same FP_EXTEND_VL for both operands.

Differential Revision: https://reviews.llvm.org/D151969