[RISCV] Optimize vfmv.s.f intrinsic with scalar 0.0 to vmv.s.x with x0.

We already do this for RISCVISD::VFMV_S_F_VL and the vfmv.v.f
intrinsic.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D121429