[compiler] Unnecessary cast warning is not detected when casting a bound generic type to its super type

[stephan-herrmann]

From https://bugs.eclipse.org/bugs/show_bug.cgi?id=517235

The following casts are unnecessary:

public class UnnecessaryCasts<T extends C> {
    void m(T t) {
        C c = t;

        ((C) t).x();
        ((I) t).x();
        ((I) c).x();
    }
}

interface I {
    void x();
}
class C implements I {
    @Override
    public void x() {}
}

Yet, the Eclipse compiler doesn't report them, and doesn't offer a quick fix to remove them.