Avoid compiler warning in non-assert builds.

After 3c90dcd03, try_partitionwise_join's child_joinrelids
variable is read only in an Assert, provoking a compiler
warning in non-assert builds.  Rearrange code to avoid the
warning and eliminate unnecessary work in the non-assert case.

Per CI testing (via Jeff Davis and Bharath Rupireddy)

Discussion: https://postgr.es/m/ef0de9713e605451f1b60b30648c5ee900b2394c.camel@j-davis.com

src/backend/optimizer/path/joinrels.c

@@ -1543,7 +1543,6 @@ try_partitionwise_join(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
 		SpecialJoinInfo *child_sjinfo;
 		List	   *child_restrictlist;
 		RelOptInfo *child_joinrel;
-		Relids		child_joinrelids;
 		AppendRelInfo **appinfos;
 		int			nappinfos;
 
@@ -1646,10 +1645,6 @@ try_partitionwise_join(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
 													 child_rel2->relids),
 										   &nappinfos);
 
-		/* Build correct join relids for child join */
-		child_joinrelids = adjust_child_relids(joinrel->relids,
-											   nappinfos, appinfos);
-
 		/*
 		 * Construct restrictions applicable to the child join from those
 		 * applicable to the parent join.
@@ -1658,8 +1653,8 @@ try_partitionwise_join(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
 			(List *) adjust_appendrel_attrs(root,
 											(Node *) parent_restrictlist,
 											nappinfos, appinfos);
-		pfree(appinfos);
 
+		/* Find or construct the child join's RelOptInfo */
 		child_joinrel = joinrel->part_rels[cnt_parts];
 		if (!child_joinrel)
 		{
@@ -1672,11 +1667,17 @@ try_partitionwise_join(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
 													child_joinrel->relids);
 		}
 
-		Assert(bms_equal(child_joinrel->relids, child_joinrelids));
+		/* Assert we got the right one */
+		Assert(bms_equal(child_joinrel->relids,
+						 adjust_child_relids(joinrel->relids,
+											 nappinfos, appinfos)));
 
+		/* And make paths for the child join */
 		populate_joinrel_with_paths(root, child_rel1, child_rel2,
 									child_joinrel, child_sjinfo,
 									child_restrictlist);
+
+		pfree(appinfos);
 	}
 }