Add for_each_from, to simplify loops starting from non-first list cells.

We have a dozen or so places that need to iterate over all but the
first cell of a List.  Prior to v13 this was typically written as
	for_each_cell(lc, lnext(list_head(list)))
Commit 1cff1b95a changed these to
	for_each_cell(lc, list, list_second_cell(list))
This patch introduces a new macro for_each_from() which expresses
the start point as a list index, allowing these to be written as
	for_each_from(lc, list, 1)
This is marginally more efficient, since ForEachState.i can be
initialized directly instead of backing into it from a ListCell
address.  It also seems clearer and less typo-prone.

Some of the remaining uses of for_each_cell() look like they could
profitably be changed to for_each_from(), but here I confined myself
to changing uses of list_second_cell().

Also, fix for_each_cell_setup() and for_both_cell_setup() to
const-ify their arguments; that's a simple oversight in 1cff1b95a.

Back-patch into v13, on the grounds that (1) the const-ification
is a minor bug fix, and (2) it's better for back-patching purposes
if we only have two ways to write these loops rather than three.

In HEAD, also remove list_third_cell() and list_fourth_cell(),
which were also introduced in 1cff1b95a, and are unused as of
cc99baa43.  It seems unlikely that any third-party code would
have started to use them already; anyone who has can be directed
to list_nth_cell instead.

Discussion: https://postgr.es/m/CAApHDvpo1zj9KhEpU2cCRZfSM3Q6XGdhzuAS2v79PH7WJBkYVA@mail.gmail.com

src/backend/commands/tablecmds.c

@@ -5732,7 +5732,7 @@ ATCheckPartitionsNotInUse(Relation rel, LOCKMODE lockmode)
 
 		inh = find_all_inheritors(RelationGetRelid(rel), lockmode, NULL);
 		/* first element is the parent rel; must ignore it */
-		for_each_cell(cell, inh, list_second_cell(inh))
+		for_each_from(cell, inh, 1)
 		{
 			Relation	childrel;
 

src/backend/nodes/nodeFuncs.c

@@ -441,7 +441,7 @@ exprTypmod(const Node *expr)
 				typmod = exprTypmod((Node *) linitial(cexpr->args));
 				if (typmod < 0)
 					return -1;	/* no point in trying harder */
-				for_each_cell(arg, cexpr->args, list_second_cell(cexpr->args))
+				for_each_from(arg, cexpr->args, 1)
 				{
 					Node	   *e = (Node *) lfirst(arg);
 
@@ -469,7 +469,7 @@ exprTypmod(const Node *expr)
 				typmod = exprTypmod((Node *) linitial(mexpr->args));
 				if (typmod < 0)
 					return -1;	/* no point in trying harder */
-				for_each_cell(arg, mexpr->args, list_second_cell(mexpr->args))
+				for_each_from(arg, mexpr->args, 1)
 				{
 					Node	   *e = (Node *) lfirst(arg);
 

src/backend/optimizer/plan/createplan.c

@@ -2261,7 +2261,7 @@ create_groupingsets_plan(PlannerInfo *root, GroupingSetsPath *best_path)
 	{
 		bool		is_first_sort = ((RollupData *) linitial(rollups))->is_hashed;
 
-		for_each_cell(lc, rollups, list_second_cell(rollups))
+		for_each_from(lc, rollups, 1)
 		{
 			RollupData *rollup = lfirst(lc);
 			AttrNumber *new_grpColIdx;

src/backend/optimizer/plan/planner.c

@@ -4430,7 +4430,7 @@ consider_groupingsets_paths(PlannerInfo *root,
 			 * below, must use the same condition.
 			 */
 			i = 0;
-			for_each_cell(lc, gd->rollups, list_second_cell(gd->rollups))
+			for_each_from(lc, gd->rollups, 1)
 			{
 				RollupData *rollup = lfirst_node(RollupData, lc);
 
@@ -4464,7 +4464,7 @@ consider_groupingsets_paths(PlannerInfo *root,
 				rollups = list_make1(linitial(gd->rollups));
 
 				i = 0;
-				for_each_cell(lc, gd->rollups, list_second_cell(gd->rollups))
+				for_each_from(lc, gd->rollups, 1)
 				{
 					RollupData *rollup = lfirst_node(RollupData, lc);
 

src/backend/parser/parse_agg.c

@@ -1083,7 +1083,7 @@ parseCheckAggregates(ParseState *pstate, Query *qry)
 
 		if (gset_common)
 		{
-			for_each_cell(l, gsets, list_second_cell(gsets))
+			for_each_from(l, gsets, 1)
 			{
 				gset_common = list_intersection_int(gset_common, lfirst(l));
 				if (!gset_common)
@@ -1774,7 +1774,7 @@ expand_grouping_sets(List *groupingSets, int limit)
 		result = lappend(result, list_union_int(NIL, (List *) lfirst(lc)));
 	}
 
-	for_each_cell(lc, expanded_groups, list_second_cell(expanded_groups))
+	for_each_from(lc, expanded_groups, 1)
 	{
 		List	   *p = lfirst(lc);
 		List	   *new_result = NIL;

src/backend/utils/adt/jsonpath_gram.y

@@ -441,7 +441,7 @@ makeItemList(List *list)
 	while (end->next)
 		end = end->next;
 
-	for_each_cell(cell, list, list_second_cell(list))
+	for_each_from(cell, list, 1)
 	{
 		JsonPathParseItem *c = (JsonPathParseItem *) lfirst(cell);
 

src/backend/utils/adt/ruleutils.c

@@ -8113,7 +8113,7 @@ get_rule_expr(Node *node, deparse_context *context,
 			{
 				BoolExpr   *expr = (BoolExpr *) node;
 				Node	   *first_arg = linitial(expr->args);
-				ListCell   *arg = list_second_cell(expr->args);
+				ListCell   *arg;
 
 				switch (expr->boolop)
 				{
@@ -8122,12 +8122,11 @@ get_rule_expr(Node *node, deparse_context *context,
 							appendStringInfoChar(buf, '(');
 						get_rule_expr_paren(first_arg, context,
 											false, node);
-						while (arg)
+						for_each_from(arg, expr->args, 1)
 						{
 							appendStringInfoString(buf, " AND ");
 							get_rule_expr_paren((Node *) lfirst(arg), context,
 												false, node);
-							arg = lnext(expr->args, arg);
 						}
 						if (!PRETTY_PAREN(context))
 							appendStringInfoChar(buf, ')');
@@ -8138,12 +8137,11 @@ get_rule_expr(Node *node, deparse_context *context,
 							appendStringInfoChar(buf, '(');
 						get_rule_expr_paren(first_arg, context,
 											false, node);
-						while (arg)
+						for_each_from(arg, expr->args, 1)
 						{
 							appendStringInfoString(buf, " OR ");
 							get_rule_expr_paren((Node *) lfirst(arg), context,
 												false, node);
-							arg = lnext(expr->args, arg);
 						}
 						if (!PRETTY_PAREN(context))
 							appendStringInfoChar(buf, ')');

src/backend/utils/adt/selfuncs.c

@@ -3519,7 +3519,7 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 		 * for remaining Vars on other rels.
 		 */
 		relvarinfos = lappend(relvarinfos, varinfo1);
-		for_each_cell(l, varinfos, list_second_cell(varinfos))
+		for_each_from(l, varinfos, 1)
 		{
 			GroupVarInfo *varinfo2 = (GroupVarInfo *) lfirst(l);