coherence errors: inspect analyse visitor causes hangs

```rust
#![recursion_limit = "128"]
trait Recur {}
impl<T> Recur for T
where
    (T, ()): Recur,
    ((), T): Recur,
{
}

trait NotImplemented {}
impl<T: NotImplemented> Recur for (T,) {}

fn main() {}
```
hangs with `-Znext-solver=coherence` due to exponential blowup somewhere.