Ignoring items instead of removing them.

In the optimizer we sometimes remove items from a query and then later, when query is finished, we add the items back in their original position to be able to do re-execution of the original query (but possible with new parameters).

Instead of removing top level items (that are always of type bool) that we may have to add back (not basic constants), we could wrap the original item with another item:
Item_removeable(original_item) of type bool
That would have 2 states: Active and true or false
When state is active, it would execute the corresponding action val(), val_bool() on the original item (like Item_ref() is doing)

• When state is 'true' then it would always return true
• When state is 'false' then it would always return false

This item would have the property:

• When created it would have state 'removed' and always return true.
• After query processing, during Item_removable->cleanup() it would change state to 'active'
• Item_removable->set_state would set the state to true or false.