LLVM assertion: Ptr must be a pointer to Val type

This code compiles fine on stable/beta, but ICEs on nightly:

``` rust
pub struct Foo {
    raw: u8
}

pub struct Bar {
    raw: u8,
}

impl Drop for Bar {
    fn drop(&mut self) {}
}

fn foo<T>() -> T { loop {} }

fn main() {
    let mut a: &mut FnMut(Foo, Bar) = foo();
    let ref mut b = a;
    b(foo(), foo());
}
```

cc @eddyb, happened last night, maybe recent trans changes?
