Remove stray references to lefttree/righttree in the executor.

The general convention in the executor is to refer to child plans
and planstates via the outerPlan[State] and innerPlan[State]
macros, but a few places didn't do it like that.  For consistency
and readability, convert all the stragglers to use the macros.
(See also commit 40f42d2a3, which did some similar cleanup a few
years ago, but missed these cases.)

Richard Guo

Discussion: https://postgr.es/m/CAMbWs4-vYhh1xsa_veah4PUed2Xq=Ed_YH3=Mqt5A3Y=EgfCEg@mail.gmail.com

src/backend/executor/nodeSetOp.c

@@ -597,6 +597,8 @@ ExecEndSetOp(SetOpState *node)
 void
 ExecReScanSetOp(SetOpState *node)
 {
+	PlanState  *outerPlan = outerPlanState(node);
+
 	ExecClearTuple(node->ps.ps_ResultTupleSlot);
 	node->setop_done = false;
 	node->numOutput = 0;
@@ -617,7 +619,7 @@ ExecReScanSetOp(SetOpState *node)
 		 * parameter changes, then we can just rescan the existing hash table;
 		 * no need to build it again.
 		 */
-		if (node->ps.lefttree->chgParam == NULL)
+		if (outerPlan->chgParam == NULL)
 		{
 			ResetTupleHashIterator(node->hashtable, &node->hashiter);
 			return;
@@ -646,6 +648,6 @@ ExecReScanSetOp(SetOpState *node)
 	 * if chgParam of subnode is not null then plan will be re-scanned by
 	 * first ExecProcNode.
 	 */
-	if (node->ps.lefttree->chgParam == NULL)
-		ExecReScan(node->ps.lefttree);
+	if (outerPlan->chgParam == NULL)
+		ExecReScan(outerPlan);
 }