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.