[llvm] [NVPTX] Add intrinsics for new narrow FP conversions (PR #173954)

[Srinivasa Ravi via llvm-commits]

Wolfram70 wrote:

> Please document these new intrinsics in NVPTXUsage.rst

Looks like none of the narrow-FP conversion intrinsics have been documented yet. So, I was thinking it would be better to document all of them together in one entry as a single (separate) change since they all follow a similar pattern and syntax. Please let me know what you think, thanks!

https://github.com/llvm/llvm-project/pull/173954