Fixup some misusages of bms_num_members()

It's a bit inefficient to test if a Bitmapset is empty by counting all the
members and seeing if that number is zero. It's much better just to use
bms_is_empty().  Likewise for checking if there are at least two members,
just use bms_membership(), which does not need to do anything more after
finding two members.

Discussion: https://postgr.es/m/CAApHDvpvwm_QjbDOb5xga%2BKmX9XkN9xQavNGm3SvDbVnCYOerQ%40mail.gmail.com
Reviewed-by: Tomas Vondra

src/backend/optimizer/path/clausesel.c

@@ -164,8 +164,7 @@ clauselist_selectivity_simple(PlannerInfo *root,
 	 * directly to clause_selectivity(). None of what we might do below is
 	 * relevant.
 	 */
-	if ((list_length(clauses) == 1) &&
-		bms_num_members(estimatedclauses) == 0)
+	if (list_length(clauses) == 1 && bms_is_empty(estimatedclauses))
 		return clause_selectivity(root, (Node *) linitial(clauses),
 								  varRelid, jointype, sjinfo);
 

src/backend/statistics/dependencies.c

@@ -1246,7 +1246,7 @@ dependencies_clauselist_selectivity(PlannerInfo *root,
 	 * of clauses. We must return 1.0 so the calling function's selectivity is
 	 * unaffected.
 	 */
-	if (bms_num_members(clauses_attnums) < 2)
+	if (bms_membership(clauses_attnums) != BMS_MULTIPLE)
 	{
 		bms_free(clauses_attnums);
 		pfree(list_attnums);
@@ -1273,18 +1273,18 @@ dependencies_clauselist_selectivity(PlannerInfo *root,
 	{
 		StatisticExtInfo *stat = (StatisticExtInfo *) lfirst(l);
 		Bitmapset  *matched;
-		int			num_matched;
+		BMS_Membership membership;
 
 		/* skip statistics that are not of the correct type */
 		if (stat->kind != STATS_EXT_DEPENDENCIES)
 			continue;
 
 		matched = bms_intersect(clauses_attnums, stat->keys);
-		num_matched = bms_num_members(matched);
+		membership = bms_membership(matched);
 		bms_free(matched);
 
 		/* skip objects matching fewer than two attributes from clauses */
-		if (num_matched < 2)
+		if (membership != BMS_MULTIPLE)
 			continue;
 
 		func_dependencies[nfunc_dependencies]