When checking if an IN predicate can be evaluated using a key
 the optimizer makes sure that all the arguments of IN are of
 the same result type. To assure that it check whether 
 Item_func_in::array is filled in. 
 However Item_func_in::array is set if the types are
 the same AND all the arguments are compile time constants.
 Fixed by introducing Item_func_in::arg_types_compatible
 flag to allow correct checking of the desired condition.