[RISCV] Define vector narrowing type-convert intrinsic.

Define intrinsics:
  1. vfncvt.xu.f.w/vfncvt.x.f.w
  2. vfncvt.rtz.xu.f.w/vfncvt.rtz.x.f.w
  3. vfncvt.f.xu.w/vfncvt.f.x.w
  4. vfncvt.f.f.w/vfncvt.rod.f.f.w

We work with @rogfer01 from BSC to come out this patch.

Authored-by: Roger Ferrer Ibanez <rofirrim@gmail.com>
Co-Authored-by: Monk Chiang <monk.chiang@sifive.com>

Differential Revision: https://reviews.llvm.org/D93932