Function from_less_existence_predicate 

fn from_less_existence_predicate(sub: &HirRelationExpr) -> Option<HirScalarExpr>

If sub is a FROM-less subquery (see simplify_from_less_existence_subqueries) whose existence check is correlated on the outer row, returns the predicate p1 AND p2 AND ... expressed in the outer row’s frame. Returns None otherwise.