Why is this compiler optimization beneficial with -ffinite-math-only and -fno-signed-integers?

From

    if (x > y) {
      do_something();
    } else {
      do_something_else();
    }

to the form

    if (x <= y) {
      do_something_else();
    } else {
      do_something();
    }

What happens when x or y are NaN?