cleanup: Item_func_case

reorder items in args[] array. Instead of

  when1,then1,when2,then2,...[,case][,else]

sort them as

  [case,]when1,when2,...,then1,then2,...[,else]

in this case all items used for comparison take a continuous part
of the array and can be aggregated directly. and all items that
can be returned take a continuous part of the array and can be
aggregated directly. Old code had to copy them to a temporary
array before aggreation, and then copy back (thd->change_item_tree)
everything that was changed.

sql/item_cmpfunc.h

@@ -1558,12 +1558,11 @@ class Item_func_case :public Item_func_hybrid_field_type
   int first_expr_num, else_expr_num;
   enum Item_result left_cmp_type;
   String tmp_value;
-  uint ncases;
+  uint nwhens;
   Item_result cmp_type;
   DTCollation cmp_collation;
   cmp_item *cmp_items[6]; /* For all result types */
   cmp_item *case_item;
-  Item **arg_buffer;
   uint m_found_types;
 public:
   Item_func_case(THD *thd, List<Item> &list, Item *first_expr_arg,
@@ -1593,7 +1592,6 @@ class Item_func_case :public Item_func_hybrid_field_type
     if (clone)
     {
       clone->case_item= 0;
-      clone->arg_buffer= 0;
       bzero(&clone->cmp_items, sizeof(cmp_items));
     }
     return clone;