The new ANTLRv4 parser generates left-deep nested Boolean AND/OR
expressions, which need to be flattened where appropriate.  The
BooleanOperator.simplify() method takes care of this operation.

Introducing this flattening/simplification everywhere all at once seemed
to be inadvisable, so currently it is only applied in the query AST when
schemas are computed.  Additionally, only WHERE predicates, HAVING
predicates, and ON clauses are simplified/flattened.  (NATURAL JOIN and
USING join predicates are generated programmatically, not by the parser,
and are already flattened.)