Fixes for WL#1724 required by the third code review.

sql/item.cc:
  Document actual behavior.
sql/opt_range.cc:
  - Added a test for range predicates comparing incomparable argumens
  - Removed TRP_GROUP_MIN_MAX class members that are not used.
  - Uncommented CPU cost
  - More standard function return values
sql/sql_select.cc:
  Remove unnecessary test.

sql/item.cc

@@ -177,9 +177,8 @@ bool Item_ident::remove_dependence_processor(byte * arg)
     arguments in a condition the method must return false.
 
   RETURN
-    false on success (force the evaluation of collect_item_field_processor
-          for the subsequent items.)
-    true  o/w (stop evaluation of subsequent items.)
+    false to force the evaluation of collect_item_field_processor
+          for the subsequent items.
 */
 
 bool Item_field::collect_item_field_processor(byte *arg)

sql/sql_select.cc

@@ -5020,8 +5020,7 @@ make_join_select(JOIN *join,SQL_SELECT *select,COND *cond)
       }
       if (tmp || !cond)
       {
-        if (tmp)
-          DBUG_EXECUTE("where",print_where(tmp,tab->table->table_name););
+        DBUG_EXECUTE("where",print_where(tmp,tab->table->table_name););
 	SQL_SELECT *sel=tab->select=(SQL_SELECT*)
 	  join->thd->memdup((gptr) select, sizeof(SQL_SELECT));
 	if (!sel)