`loop` can mask state changes in typestate

[paulstansifer]

The following (compile-fail/loop-pred-constraints.rs) should not compile:

fn print_even(y: int) : even(y) { log(debug, y); }
pure fn even(y: int) -> bool { true }

fn main() {
    let mut y = 42;
    check (even(y));
    loop {
        print_even(y);
        loop { y += 1; break; }
    }
}

Removing the inner loop or changing it to a do (back when `do existed) gives the correct error.