Treat OpFilter as it's own type of Visitor, not simply an Op1, 
because Ops can occur in expressions (e.g. NOT EXISTS).
This refactoring does not change any banhaviour but does
highlight that fact that transformations need to be aware
of this fact. See also ApplyTransformVisitor.

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1441897 13f79535-47bb-0310-9956-ffa450edef68