[llvm] [NVPTX] Support copysign PTX instruction (PR #107800)

[Artem Belevich via llvm-commits]

https://github.com/Artem-B approved this pull request.

LGTM.

For my own education -- is there hardware support for this instruction in SASS? Or would the copysign get expanded to the bit twiddling LLVM does?

https://github.com/llvm/llvm-project/pull/107800