ICE: `adt::represent_type called on non-ADT type` when returning during tuple initialization

Today is not my day! I was trying to create a test case for another bug and I bumped into this ICE:

``` rust
fn foo() -> (int, int) {
  (0, return (0, 0))
}

fn main() {}
```

I don't think this is a useful pattern (the original involved a `try!` which I had simplified to `return` just for the sake of bug reproduction --- with `try!` it doesn't ICE) but it probably shouldn't ICE.
