greedy_search() and best_extension_by_limited_search() scrambled table order

best_extension_by_limited_search() assumes that tables should be sorted
according to size to be able to quickly disregard bad plans. However the
current usage of swap_variables() will change the table order to a not
sorted one for the next recursive call. This breaks the assumtion and
causes performance issues when using many tables (we have to examine
many more plans).

This patch fixes this by ensuring that the original table order is kept
for the not yet used tables when best_extension_by_limited_search() is
called.

This was done by always calling swap_variables() for each table and
restoring the original table order at exit.

Some test changed:
- In a majority of the test the change was that two "identical tables"
  where swapped and the optimzer is now using the first/smaller table
- In few test the table order was changed. The new plan looks identical
  or slighly better than the original.

mysql-test/main/selectivity.result

@@ -326,8 +326,8 @@ group by c_name, c_custkey, o_orderkey, o_orderdate, o_totalprice
 order by o_totalprice desc, o_orderdate;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	orders	ALL	PRIMARY,i_o_custkey	NULL	NULL	NULL	1500	100.00	Using where; Using temporary; Using filesort
-1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	4	dbt3_s001.orders.o_orderkey	1	100.00	
 1	PRIMARY	customer	eq_ref	PRIMARY	PRIMARY	4	dbt3_s001.orders.o_custkey	1	100.00	
+1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	4	dbt3_s001.orders.o_orderkey	1	100.00	
 1	PRIMARY	lineitem	ref	PRIMARY,i_l_orderkey,i_l_orderkey_quantity	i_l_orderkey_quantity	4	dbt3_s001.orders.o_orderkey	4	100.00	Using index
 2	MATERIALIZED	lineitem	index	NULL	i_l_orderkey_quantity	13	NULL	6005	100.00	Using index
 Warnings:
@@ -360,8 +360,8 @@ group by c_name, c_custkey, o_orderkey, o_orderdate, o_totalprice
 order by o_totalprice desc, o_orderdate;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	orders	ALL	PRIMARY,i_o_custkey	NULL	NULL	NULL	1500	100.00	Using where; Using temporary; Using filesort
-1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	4	dbt3_s001.orders.o_orderkey	1	100.00	
 1	PRIMARY	customer	eq_ref	PRIMARY	PRIMARY	4	dbt3_s001.orders.o_custkey	1	100.00	
+1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	4	dbt3_s001.orders.o_orderkey	1	100.00	
 1	PRIMARY	lineitem	ref	PRIMARY,i_l_orderkey,i_l_orderkey_quantity	i_l_orderkey_quantity	4	dbt3_s001.orders.o_orderkey	4	100.00	Using index
 2	MATERIALIZED	lineitem	index	NULL	i_l_orderkey_quantity	13	NULL	6005	100.00	Using index
 Warnings: