SI-9044 Fix order of interfaces in classfiles

It was reversed since ced3ca8ae1. The reason is that the backend used
`mixinClasses` to obtain the parents of a class, which returns them in
linearization order.

`mixinClasses` als returns all ancestors (not only direct parents),
which means more work for `minimizeInterfaces`. This was introduced
in cd62f52 for unclear reasons. So we switch back to using `parents`.
Merge pull request #4208 from lrytz/t9044

SI-9044 Fix order of interfaces in classfiles