MDEV-8989: ORDER BY optimizer ignores equality propagation

Variant #4 of the fix.

Make ORDER BY optimization functions take into account multiple
equalities. This is done in several places:
- remove_const() checks whether we can sort the first table in the
  join, or we need to put rows into temp.table and then sort.
- test_if_order_by_key() checks whether there are indexes that
  can be used to produce the required ordering
- make_unireg_sortorder() constructs sort criteria for filesort.

sql/sql_select.h

@@ -1821,7 +1821,9 @@ bool error_if_full_join(JOIN *join);
 int report_error(TABLE *table, int error);
 int safe_index_read(JOIN_TAB *tab);
 int get_quick_record(SQL_SELECT *select);
-SORT_FIELD * make_unireg_sortorder(THD *thd, ORDER *order, uint *length,
+SORT_FIELD *make_unireg_sortorder(THD *thd, JOIN *join,
+                                  table_map first_table_map,
+                                  ORDER *order, uint *length,
                                   SORT_FIELD *sortorder);
 int setup_order(THD *thd, Item **ref_pointer_array, TABLE_LIST *tables,
 		List<Item> &fields, List <Item> &all_fields, ORDER *order);