This is relic code when we had PermGen. ConstantPoolCache is now a direct subclass of MetaspaceObj so it doesn't have a vtable. Also, it has no subclasses, so this function serves no purposes.

./share/oops/cpCache.cpp: assert(is_constantPoolCache(), "obj must be constant pool cache");
./share/oops/cpCache.cpp: assert(is_constantPoolCache(), "obj must be constant pool cache");
./share/oops/cpCache.cpp: guarantee(is_constantPoolCache(), "obj must be constant pool cache");
./share/oops/constantPool.cpp: guarantee(cache()->is_constantPoolCache(), "should be constant pool cache");
./share/oops/cpCache.hpp: bool is_constantPoolCache() const { return true; }