[RISCV] Define vector widening type-convert intrinsic.

Define intrinsics:
  1. vfwcvt.xu.f.v/vfwcvt.x.f.v
  2. vfwcvt.rtz.xu.f.v/vfwcvt.rtz.x.f.v
  3. vfwcvt.f.xu.v/vfwcvt.f.x.v
  4. vfwcvt.f.f.v

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/D93855