Refactoring of the cost model:

- Dynamic is now an invalid value
 - introduce a HugeCost constant to be used for runtime-cost values or arbitrarily huge cost
 - add sanity checks for cost values: must be >=0 and not too large
This change provides several benefits:
 - it fixes shortcoming is some cost computation where the Dynamic case was not properly handled.
 - it simplifies cost computation logic, and should avoid future similar shortcomings.
 - it allows to distinguish between different level of dynamic/huge/infinite cost
 - it should enable further simplifications in the computation of costs (save compilation time)

unsupported/Eigen/src/KroneckerProduct/KroneckerTensorProduct.h

@@ -240,7 +240,7 @@ struct traits<KroneckerProductSparse<_Lhs,_Rhs> >
 
     Flags = ((LhsFlags | RhsFlags) & HereditaryBits & RemovedBits)
           | EvalBeforeNestingBit | EvalBeforeAssigningBit,
-    CoeffReadCost = Dynamic
+    CoeffReadCost = HugeCost
   };
 
   typedef SparseMatrix<Scalar, 0, StorageIndex> ReturnType;

unsupported/Eigen/src/Skyline/SkylineProduct.h

@@ -49,7 +49,7 @@ struct internal::traits<SkylineProduct<LhsNested, RhsNested, ProductMode> > {
         | EvalBeforeAssigningBit
         | EvalBeforeNestingBit,
 
-        CoeffReadCost = Dynamic
+        CoeffReadCost = HugeCost
     };
 
     typedef typename internal::conditional<ResultIsSkyline,