Fix test for subplans in force-parallel mode.

We mustn't force parallel mode if the query has any subplans, since
ExecSerializePlan doesn't transmit them to workers.  Testing
top_plan->initPlan is inadequate because (1) there might be initPlans
attached to lower plan nodes, and (2) non-initPlan subplans don't
work either.  There's certainly room for improvement in those
restrictions, but for the moment that's what we've got.

Amit Kapila, per report from Andreas Seltenreich

Discussion: <8737im6pmh.fsf@credativ.de>

src/backend/optimizer/plan/planner.c

@@ -342,11 +342,11 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
 	 * actually a safe thing to do.  (Note: we assume adding a Material node
 	 * above did not change the parallel safety of the plan, so we can still
 	 * rely on best_path->parallel_safe.  However, that flag doesn't account
-	 * for initPlans, which render the plan parallel-unsafe.)
+	 * for subplans, which we are unable to transmit to workers presently.)
 	 */
 	if (force_parallel_mode != FORCE_PARALLEL_OFF &&
 		best_path->parallel_safe &&
-		top_plan->initPlan == NIL)
+		glob->subplans == NIL)
 	{
 		Gather	   *gather = makeNode(Gather);