Bit operators (~ ^ | & << >>) and the function BIT_COUNT()
always called val_int() for their arguments.
It worked correctly only for INT type arguments.

In case of DECIMAL and DOUBLE arguments it did not work well:
the argument values were truncated to the maximum SIGNED BIGINT value
of 9223372036854775807.

Fixing the code as follows:

- If the argument if of an integer data type,
  it works using val_int() as before.

- If the argument if of some other data type, it gets the argument value
  using val_decimal(), to avoid truncation, and then converts the result
  to ulonglong.

Using Item_handled_func to switch between the two approaches easier.

As an additional advantage, with Item_handled_func it will be easier
to implement overloading in the future, so data type plugings will be able
to define their own behavioir of bit operators and BIT_COUNT().

Moving the code from the former val_int() implementations
as methods to Longlong_null, to avoid code duplication in the
INT and DECIMAL branches.