[NVPTX] Add more FMA intriniscs/builtins

This patch adds builtins/intrinsics for the following variants of FMA:

NOTE: follow-up commit with the missing clang-side changes.

- f16, f16x2
  - rn
  - rn_ftz
  - rn_sat
  - rn_ftz_sat
  - rn_relu
  - rn_ftz_relu
- bf16, bf16x2
  - rn
  - rn_relu

ptxas (Cuda compilation tools, release 11.0, V11.0.194) is happy with the generated assembly.

Differential Revision: https://reviews.llvm.org/D118977