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.

this is a 10.3 version of bf1ca14f

mysql-test/main/func_debug.result

@@ -1630,7 +1630,7 @@ c
 NULL
 Warnings:
 Note	1105	DBUG: [0] arg=1 handler=0 (bigint)
-Note	1105	DBUG: [1] arg=3 handler=1 (decimal)
+Note	1105	DBUG: [1] arg=2 handler=1 (decimal)
 DROP TABLE t1;
 #
 # MDEV-11555 CASE with a mixture of TIME and DATETIME returns a wrong result
@@ -1649,9 +1649,9 @@ good	was_bad_now_good
 one	one
 Warnings:
 Note	1105	DBUG: [0] arg=1 handler=0 (time)
-Note	1105	DBUG: [1] arg=3 handler=0 (time)
+Note	1105	DBUG: [1] arg=2 handler=0 (time)
 Note	1105	DBUG: [0] arg=1 handler=0 (time)
-Note	1105	DBUG: [1] arg=3 handler=0 (time)
-Note	1105	DBUG: [2] arg=5 handler=2 (datetime)
+Note	1105	DBUG: [1] arg=2 handler=0 (time)
+Note	1105	DBUG: [2] arg=3 handler=2 (datetime)
 SET SESSION debug_dbug="-d,Predicant_to_list_comparator";
 SET SESSION debug_dbug="-d,Item_func_in";

sql/item_cmpfunc.h

@@ -2104,15 +2104,13 @@ class Item_func_case :public Item_func_case_expression
 protected:
   String tmp_value;
   DTCollation cmp_collation;
-  Item **arg_buffer;
-  bool aggregate_then_and_else_arguments(THD *thd,
-                                         Item **items, uint count,
-                                         Item **else_expr);
+  bool aggregate_then_and_else_arguments(THD *thd, uint count);
   virtual Item **else_expr_addr() const= 0;
   virtual Item *find_item()= 0;
   void print_when_then_arguments(String *str, enum_query_type query_type,
                                  Item **items, uint count);
   void print_else_argument(String *str, enum_query_type query_type, Item *item);
+  void reorder_args(uint start);
 public:
   Item_func_case(THD *thd, List<Item> &list)
    :Item_func_case_expression(thd, list)
@@ -2129,13 +2127,6 @@ class Item_func_case :public Item_func_case_expression
   enum precedence precedence() const { return BETWEEN_PRECEDENCE; }
   CHARSET_INFO *compare_collation() const { return cmp_collation.collation; }
   bool need_parentheses_in_default() { return true; }
-  Item *build_clone(THD *thd)
-  {
-    Item_func_case *clone= (Item_func_case *) Item_func::build_clone(thd);
-    if (clone)
-      clone->arg_buffer= 0;
-    return clone;
-  }
 };
 
 
@@ -2156,6 +2147,7 @@ class Item_func_case_searched: public Item_func_case
    :Item_func_case(thd, list)
   {
     DBUG_ASSERT(arg_count >= 2);
+    reorder_args(0);
   }
   void print(String *str, enum_query_type query_type);
   void fix_length_and_dec();
@@ -2200,6 +2192,7 @@ class Item_func_case_simple: public Item_func_case,
     m_found_types(0)
   {
     DBUG_ASSERT(arg_count >= 3);
+    reorder_args(1);
   }
   void cleanup()
   {
@@ -2233,8 +2226,7 @@ class Item_func_decode_oracle: public Item_func_case_simple
    :Item_func_case_simple(thd, list)
   { }
   const char *func_name() const { return "decode_oracle"; }
-  void print(String *str, enum_query_type query_type)
-  { Item_func::print(str, query_type); }
+  void print(String *str, enum_query_type query_type);
   void fix_length_and_dec();
   Item *find_item();
   Item *get_copy(THD *thd)