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/nodeProjectSet.c

@@ -339,6 +339,8 @@ ExecEndProjectSet(ProjectSetState *node)
 void
 ExecReScanProjectSet(ProjectSetState *node)
 {
+	PlanState  *outerPlan = outerPlanState(node);
+
 	/* Forget any incompletely-evaluated SRFs */
 	node->pending_srf_tuples = false;
 
@@ -346,6 +348,6 @@ ExecReScanProjectSet(ProjectSetState *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);
 }