make slicing context sensitive precise

[mchalupa]

we have interprocedural edges that make strong updates impossible. For example in this code:

int a;

void foo(void)
{
        a = 3;
}

int main(void)
{
        a = 1;
        foo();
        foo();
        test_assert(a == 3); 

        return 0;
}

we should keep only the last foo() call and remove the a = 1 and the other foo call, but we keep everything (with both old and new pta analysis)

[mchalupa]

yeah, but that needs better slicing algorithm (now we make all call-sites a dependence for the called function, so when we keep the function, we keep all callsites...)

[mchalupa]

Subissue of #242

[mchalupa]

See also #428 .