If all elements in the list of 'IN' or 'NOT IN' clause are equal
and there are no NULLs then clause
-  "a IN (e1,..,en)" can be converted to "a = e1"
-  "a NOT IN (e1,..,en)" can be converted to "a <> e1".
This means an object of Item_func_in can be replaced with an object
of Item_func_eq for IN (e1,..,en) clause and Item_func_ne for
NOT IN (e1,...,en). Such a replacement allows the optimizer to choose
a better execution plan