Every single time you run those two statements, you’ll get the same result. Yes they're non-associative. But that's specified and documented. That's not the same thing as non-deterministic in any way.
It's not an optimisation if it changes the result! And if you use non-standard flags that's your problem.
MP3 is an optimization of WAV, yet it changes the result.
Some applications are ok with reducing precision of calculations because they are not sensitive enough to small inaccuracies or they take effort to control inaccuarcies.
For example, graphics applications are typically heavy in FP calculations and yet they tend to not care much about precision and much more about performance. For those applications reducing accuracy for slight performance increase is likely win.