IN/BETWEEN predicates in sorting expressions.
Wrong results may occur when the select list contains an expression
with IN/BETWEEN predicate that differs from a sorting expression by
an additional NOT only.
 
Added the method Item_func_opt_neg::eq to compare correctly expressions
containing [NOT] IN/BETWEEN.
The eq method inherited from the Item_func returns TRUE when comparing
'a IN (1,2)' with 'a NOT IN (1,2)' that is not, of course, correct.