This patch fixes Trac #12175, another delicate corner case of
Note [Instance and Given overlap] in TcInteract.

In #12175 we were not expanding given superclasses eagerly
enough. It was easy to fix, and is actually rather neater than
before.

See Note [Eagerly expand given superclasses] in TcCanonical.
The main change is to move the eager expansion of given superclasses
to canClassNC.