Fix false unreachable error reports. - #9909

So far, method `TypeChecker.intersect_instances` generally reports an unreachable error for statements like `isinstance(a, B)` when `class C(A, B)` is unsafe without checking if `class C(B, A)` could be safe.

This commit solves this by performing the "reversed" check only if the usual one fails.  Hence, other functionalities and performance should not be affected.

The new test case `testIsInstanceAdHocIntersectionReversed` demonstrates the new functionality.

Note that this commit changes the type of argument `instances`.  It is now `Tuple[Instance, Instance]` instead of `Sequence[Instance]`.  Supporting an arbitrary number of `Instance` objects would require stronger code modifications.