[llvm] [InstCombine] Fix FMF propagation in `copysign Mag, (copysign ?, X) -> copysign Mag, X` (PR #121574)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

Can we create a new call here instead of modifying the operand? Otherwise there would still be a potential problem with nofpclass on the argument.

https://github.com/llvm/llvm-project/pull/121574