[NVPTX] Added support for half-precision floating point.

[NVPTX] Added support for half-precision floating point.

Only scalar half-precision operations are supported at the moment.

- Adds general support for 'half' type in NVPTX.
- fp16 math operations are supported on sm_53+ GPUs only
  (can be disabled with --nvptx-no-f16-math).
- Type conversions to/from fp16 are supported on all GPU variants.
- On GPU variants that do not have full fp16 support (or if it's disabled),
  fp16 operations are promoted to fp32 and results are converted back
  to fp16 for storage.

Differential Revision: https://reviews.llvm.org/D28540

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291956 91177308-0d34-0410-b5e6-96231b3b80d8