Don't allow instanceof @Nullable C

[wmdietl]

instanceof is only true for a non-null expression https://docs.oracle.com/javase/specs/jls/se13/html/jls-15.html#jls-15.20.2.

Code like this:

if (nbl instanceof @Nullable C)

is misleading: the then-branch is not taken for a null value.

A @Nullable annotation on the type of an instanceof should raise an error.
Having a @NonNull annotation is redundant, and could cause a warning.

[smillst]

Related issue: #762