[patch, fortran] Really fix PR 56782

[Thomas Koenig]

Hi Mikael,

>>
>> -  (void) gfc_expr_walker (&fcn, callback_reduction, NULL);
>
> why remove this?

Because it is not needed, as the test case _46 shows.  No need
to run this twice, it doesn't get better :-)

It is a leftover from when the callback function returned 1.

   gfc_internal_error ("Illegal id in insert_iterator_function");
>
> This duplicated code could probably be merged with
> copy_walk_reduction_arg.

I thought about it. The reason why I didn't do it was
because the expr to be wrapped inside the call is different.
Another reason was that I didn't get it right when I tried
this approach.
So, I'd rather leave it as is.

I think callback_reduction's iterator handling
> should happen there as well.

Like I said, it is done automatically by the expression
walker.

	Thomas