message: Flip not negate
diff: @@ -686,7 +686,7 @@ pair<RelationSet*, unique_ptr<LogicalOperator>> JoinOrderOptimizer::GenerateJoin
  cond.comparison = condition->type;
  if (left_child != 0) {
  // reverse comparison expression if we reverse the order of the children
  - cond.comparison = ComparisonExpression::NegateComparisionExpression(cond.comparison);
  + cond.comparison = ComparisonExpression::FlipComparisionExpression(cond.comparison);
  }
  join->conditions.push_back(move(cond));
  }
  