[Bug tree-optimization/126745] a/b -> 1 if it is known that `a==b` in VRP without DOM

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126745

--- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> ---
Need to handle % too.

That is:
```
int f1(int a, int b)
{
    if (a != b) __builtin_unreachable();
    return a%b;
}
```
is 0.