internal compiler error: Got a fat pointer where a scalar was expected

The following code causes an internal error in miri:

```rust
use std::rc::Rc;
use std::fmt::Debug;

fn main() {
    let p = Rc::new(1) as Rc<Debug>;
    let a: *const Debug = &*p;
    let r = Rc::into_raw(p);
    a == r;
}
```

```
error: internal compiler error: src/librustc_mir/interpret/operand.rs:73: Got a fat pointer where a scalar was expected
```