select result

Item equal objects are employed only at the optimize phase. Usually they are not
supposed to be evaluated.  Yet in some cases we call the method val_int() for
them. Here we have to take care of restricting the predicate such an object
represents f1=f2= ...=fn to the projection of known fields fi1=...=fik.

Added a check for field's table being const in Item_equal::val_int().
If the field's table is not const val_int() just skips that field when
evaluating Item_equal.


mysql-test/t/select.test:
  Added test case for bug #15633: Evaluation of Item_equal for non-const table caused wrong select result
mysql-test/r/select.result:
  Added test case for bug #15633: Evaluation of Item_equal for non-const table caused wrong select result
mysql-test/r/func_group.result:
  Corrected test result for bug #12882 after fix for bug#15633
sql/item_cmpfunc.h:
  Fixed bug #15633: Evaluation of Item_equal for non-const table caused wrong select result
  Added comment about fields from non-const tables in class description.
sql/item_cmpfunc.cc:
  Fixed bug #15633: Evaluation of Item_equal for non-const table caused wrong select result
  Added check for field's table being const in Item_equal::val_int().