"ref" in guarded patterns trips LLVM asserts

This crashes if you try to compile it:

```
fn main() {
    let x = some(unsafe::exclusive(false));
    match move x {
        some(ref z) if z.with(|b| *b) => {
            do z.with |b| { assert *b; }
        },
        _ => fail
    }
}
```

Related #2329, #3255
