use Item->neg to convert generate negative Item_num's
instead of Item_func_neg(Item_num).

Based on the following commit:

  Author: Monty <monty@mariadb.org>
  Date:   Mon May 30 22:44:00 2016 +0300

    Make negative number their own token
    The negation (-) operator will call Item->neg() one underlying numeric constants
    and remove itself (like the NOT() function does today for other NOT functions.

    This simplifies things
    - -1 is not anymore an expression but a basic_const_item
      - improves optimizer
      - DEFAULT -1 doesn't need special handling anymore
      - When we add DEFAULT expressions, -1 will be treated exactly like 1
    - printing of items doesn't anymore put braces around all negative numbers

    Other things fixed:
    - Fixed that longlong converted to decimal's has a more appropriate size
    - Fixed that "-0.0" read into a decimal is interpreted as 0.0