"doesn't affect other things" is in no way ever whatsoever a reason to do anything in any way related to thinking, believing, or hoping that it might, would, or should affect nothing, especially around compiler optimizations (fun sentence to write, and one I violate myself for some things, but trivially true regardless). I'd be curious to hear about actual issues though.

The ppc64 case looks like llvm very intentionally not using the existing square root instruction, instead emitting a sequence of manual operations that supposedly run faster. And it's entirely in its right to do so, and it should affect no correct code (not that it's even really possible to write "correct code" under -ffast-math).