Implement ORDER BY DESC optimization, which reads values in descending

order directly from the index instead of using a filesort.

sql/opt_range.h

@@ -80,6 +80,20 @@ public:
   bool unique_key_range();
 };
 
+class QUICK_SELECT_DESC: public QUICK_SELECT
+{
+public:
+  QUICK_SELECT_DESC(QUICK_SELECT *q);
+  int get_next();
+private:
+  int cmp_prev(QUICK_RANGE *range);
+  bool range_reads_after_key(QUICK_RANGE *range);
+
+  QUICK_SELECT *quick;
+  List<QUICK_RANGE> rev_ranges;
+  List_iterator<QUICK_RANGE> rev_it;
+};
+
 class SQL_SELECT :public Sql_alloc {
  public:
   QUICK_SELECT *quick;		// If quick-select used