[lno] fix division by zero

[Daniel Berlin]

>
>> It appears it doesn't dump why vectorization did or did not occur, 
>> even
>> with -details.
>
> At the end of tree-vectorizer.h you can set
> #define DBG_VECT(A)
> to
> #define DBG_VECT(A) A
> to get information on why vectorization did not occur.
> Setting
> #define DBG_VECT2(A) A
> will spit even more details.
>
> It should be changed to be cotrolled by a flag.

Yeah, i'm considering adding TDF_debug to go along with TDF_details and 
TDF_stats

>
> dorit
>