Without this it was possible (at least on the incorrect code) that boxed and unboxed classes for the same value get united, which in turn caused repeating entry in DfaMemoryStateImpl#myIdToEqClassesIndices, which could cause NPE when performing successive flush for given variable.
Now we throw directly at classes unification attempt to catch illegal state earlier
Fixes EA-118152 - IAE: EqClass.$$$reportNull$$$
