[llvm] [NVPTX] designate fabs and fneg as free (PR #121513)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> This looks like a nice improvement, but I'm afraid it's built on a wrong assumption. PTX `abs` and `neg` are not equivalent to LLVM's `fabs` and `fneg` since the latter explicitly preserve NaN bits while PTX instructions do not.
> 
> LLVM uses that assumption internally so even if the user doesn't care about NaN payloads in general, it can miscompile things. https://godbolt.org/z/Y9Pr9jGxe

This isn't new, nor is it necessarily a problem we need to fix. For as long as the NVPTX backend has existed we've lowered llvm `fabs` and `fneg` to PTX `abs` and `neg` even though the semantics don't quite match. This is unfortunate but likely the least bad option, as lowering them with `and` / `xor` would significantly harm performance in most cases and fix correctness only for some rare cases that no one seems to really care about. 

@d0k Is this change causing an issue in a real program?

https://github.com/llvm/llvm-project/pull/121513