[RISCV] Define vector single-width type-convert intrinsic.

Define intrinsics:
  1. vfcvt.xu.f.v/vfcvt.x.f.v
  2. vfcvt.rtz.xu.f.v/vfcvt.rtz.x.f.v
  3. vfcvt.f.xu.v/vfcvt.f.x.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/D93933