Infinite loop when inlining

```rust
fn main() {
    f();
}

#[inline]
fn a<T: FnOnce()>(t: T) {
    t();
}

#[inline]
fn f() {
    a(f);
}
```

```console
$ rustc -Zmir-opt-level=2 a.rs
...
```

It was already possible before, but #68965 made it much easier to trigger.

@rustbot modify labels: +A-mir-opt, +A-mir-opt-inlining 