MDEV-13864 (partial) Change Item_func_case to store the predicant in args[0]

This is a preparatory step for MDEV-13864.
It does not change behavior in any ways. It simply splits methods into smaller peaces.
The intent of this separate patch is to make more readable the main patch for
MDEV-13864 (which will actually move the predicant to args[0]).

1. Splitting fix_length_and_dec() into smaller pieces, adding:
  - bool aggregate_then_and_else_arguments(THD *thd);
  - bool aggregate_switch_and_when_arguments(THD *thd);

2. Splitting find_item() into smaller pieces, adding:
  - Item *find_item_searched();
  - Item *find_item_simple();

3. Splitting print() into smaller pieces, adding:

  - 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)

4. Moving the maybe_null handling part related to ELSE from fix_length_and_dec()
   to fix_fields(), as in all other Item_func's.

5. Removing the unused String* argument from find_item().

6. Moving find_item() from public to private, as it's not needed outside.

sql/item_cmpfunc.h

@@ -2041,6 +2041,17 @@ class Item_func_case :public Item_func_case_expression,
   Item **arg_buffer;
   uint m_found_types;
   bool prepare_predicant_and_values(THD *thd, uint *found_types);
+  bool aggregate_then_and_else_arguments(THD *thd);
+  bool aggregate_switch_and_when_arguments(THD *thd);
+  Item *find_item_searched();
+  Item *find_item_simple();
+  Item *find_item()
+  {
+    return first_expr_num == -1 ? find_item_searched() : find_item_simple();
+  }
+  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);
 public:
   Item_func_case(THD *thd, List<Item> &list, Item *first_expr_arg,
                  Item *else_expr_arg);
@@ -2055,7 +2066,6 @@ class Item_func_case :public Item_func_case_expression,
   const char *func_name() const { return "case"; }
   enum precedence precedence() const { return BETWEEN_PRECEDENCE; }
   virtual void print(String *str, enum_query_type query_type);
-  Item *find_item(String *str);
   CHARSET_INFO *compare_collation() const { return cmp_collation.collation; }
   void cleanup()
   {
@@ -2081,6 +2091,7 @@ class Item_func_case :public Item_func_case_expression,
   } 
 };
 
+
 /*
   The Item_func_in class implements
   in_expr IN (<in value list>)