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.


mysql-test/r/func_in.result:
  BUG#20420: optimizer reports wrong keys on left join with IN
   - reenabled the test case
mysql-test/t/func_in.test:
  BUG#20420: optimizer reports wrong keys on left join with IN
   - reenabled the test case
sql/item_cmpfunc.cc:
  BUG#20420: optimizer reports wrong keys on left join with IN
   Check the IN argument types in a correct way
sql/item_cmpfunc.h:
  BUG#20420: optimizer reports wrong keys on left join with IN
   Check the IN argument types in a correct way
sql/opt_range.cc:
  BUG#20420: optimizer reports wrong keys on left join with IN
   Check the IN argument types in a correct way