We should warn on a bound + compute_at combination that makes no sense

These are easy mistakes to make, and they blow up the computational complexity of the algorithm:

```
Func f, g;
f(x) = x;
g(x) = f(x);

f.bound(x, 0, width); // Added while everything was compute_root
g.bound(x, 0, width);

f.compute_at(g, x); // Added later, but oops, this makes the bound silly

```