Fix unexpected circular dependency check

The PruneObjectReferenceVisitor, responsible for traversal instances of
a class, to avoid circular dependenceis which makes infinite recursion,
should only record an object received if it's not a ClassClass, because
the processing of ClassClass objects will be forwarded to
PruneAppImageClassInternal, which will first check circular
dependencies.

Test: make test-art-host -j64
Change-Id: Ib056d3fcd02135992b09f7d28176da3a1173d42e