Field sensitivity lost with nested structs.

In this example the graph contains a dependency from the old `img.user` when the field is overwritten.

```rs
struct User {
    name: String,
}

struct Image {
    user: User,
    name: String,
}

for mut img in Image::for_user(&user) {
    img.user = User {
        name: "dummy".to_string(),
    };
    img.name = "dummy".to_string();
    img.delete()
}
```