Patch to support mark_constant_function marking pure functions

[Jan Hubicka]

>   static int
>   subr (int a[3], int b[3])
>     {
>     int c;
>     int i;
>     int j;
> 
>     c = 0;
>     for (i = 0; i < 3; i++)
>       for (j = 0; j < 3; j++)
>         c += a[i] * b[j];
>     }
> 
> is a pure function.
It should not be too long term now, as loop optimizer marks every loop
with known iteration counts by "loop iterations" heuiristics, so theoretically
all you need to do is to verify that each loop contains such note IMO.

Honza
> 
> -- John
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