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/execCurrent.c

@@ -396,7 +396,7 @@ search_plan_tree(PlanState *node, Oid table_oid,
 			 */
 		case T_ResultState:
 		case T_LimitState:
-			result = search_plan_tree(node->lefttree,
+			result = search_plan_tree(outerPlanState(node),
 									  table_oid,
 									  pending_rescan);
 			break;