Evaluate dif for inequalities only if arguments are reals

fixes #8288
hyperexpand is for working out integrals: it has a few tests that show the things like x, x + 1 should order, it uses "sorting by difference (`items.sort(key=lambda x: x - x0, reverse=flip)`). Although it should recast variables probably as rationals, it gets around the issue by using differences so I think the changes you have made there are fine.