Cosmetic improvements in setup of planner's per-RTE arrays.

Merge setup_append_rel_array into setup_simple_rel_arrays.  There's no
particularly good reason to keep them separate, and it's inconsistent
with the lack of separation in expand_planner_arrays.  The only apparent
benefit was that the fast path for trivial queries in query_planner()
doesn't need to set up the append_rel_array; but all we're saving there
is an if-test and NULL assignment, which surely ought to be negligible.

Also improve some obsolete comments.

Discussion: https://postgr.es/m/17220.1565301350@sss.pgh.pa.us

src/backend/optimizer/prep/prepunion.c

@@ -132,16 +132,10 @@ plan_set_operations(PlannerInfo *root)
 	/*
 	 * We'll need to build RelOptInfos for each of the leaf subqueries, which
 	 * are RTE_SUBQUERY rangetable entries in this Query.  Prepare the index
-	 * arrays for that.
+	 * arrays for those, and for AppendRelInfos in case they're needed.
 	 */
 	setup_simple_rel_arrays(root);
 
-	/*
-	 * Populate append_rel_array with each AppendRelInfo to allow direct
-	 * lookups by child relid.
-	 */
-	setup_append_rel_array(root);
-
 	/*
 	 * Find the leftmost component Query.  We need to use its column names for
 	 * all generated tlists (else SELECT INTO won't work right).