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 bf1ca14

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";