XOR conditions are not optimized, and Item_cond_xor therefore
acts like type Func_item even though it inherits from Item_cond.
A subtle difference between Item_func and Item_cond is that
you can get the children Items from the former by calling
arguments(), and from the latter by calling argument_list().
However, since Item_cond_xor inherits from Item_cond,
arguments() did not return any Items.

The fact that Item_cond_xor::arguments() did not return it's
children items lead to a problem for make_cond_for_index();
the method accepted that XOR items on unindexed columns were
pushed using ICP. ICP evaluation of non-indexed columns
does not (and should not) work.

The fix for this bug is to make Item_cond_xor return it's
children items when the arguments() method is used. This makes
Item_cond_xor behave more like Item_func and in turn allows
make_cond_for_index() to discover any conflicting children
Items.

This is a temporary fix and should be removed when Item_cond_xor
 is optimized.