Refactor computeProductBlockingSizes to make room for the possibility of using lookup tables

Eigen/src/Core/util/ForwardDeclarations.h

@@ -287,6 +287,14 @@ struct stem_function
   typedef std::complex<typename NumTraits<Scalar>::Real> ComplexScalar;
   typedef ComplexScalar type(ComplexScalar, int);
 };
+
+template <typename LhsScalar,
+          typename RhsScalar>
+struct BlockingSizesLookupTable
+{
+  static const size_t NumSizes = 0;
+};
+
 }
 
 } // end namespace Eigen