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/nodeGatherMerge.c

@@ -212,13 +212,13 @@ ExecGatherMerge(PlanState *pstate)
 
 			/* Initialize, or re-initialize, shared state needed by workers. */
 			if (!node->pei)
-				node->pei = ExecInitParallelPlan(node->ps.lefttree,
+				node->pei = ExecInitParallelPlan(outerPlanState(node),
 												 estate,
 												 gm->initParam,
 												 gm->num_workers,
 												 node->tuples_needed);
 			else
-				ExecParallelReinitialize(node->ps.lefttree,
+				ExecParallelReinitialize(outerPlanState(node),
 										 node->pei,
 										 gm->initParam);