Refactored reducer so that the 'finding' functionality of a reduction pass are...

Refactored reducer so that the 'finding' functionality of a reduction pass are separated from the generic functionality for tracking progress of a pass.  With this change, we now have a ReductionOpportunityFinder abstract class, with many subclasses for each type of reduction, and just one ReductionPass class, which has an associated finder. (#2321)

Sounds good.