[llvm] [NVPTX] Add support for atomic add for f16 type (PR #84295)

[Adrian Kuegel via llvm-commits]

akuegel wrote:

Ok, seems that ptx makes a difference between floating point constants and integer constants. And we are generating a integer constant, I guess due to using Int16Register. @Artem-B in case you can give me a pointer where I can make sure that we are using a floating point constant here, that would be great :)

https://github.com/llvm/llvm-project/pull/84295