"while(i!=-1)" causes "Unsatisfied precondition constraint"

```
$ cat ~/tmp/test.rs
fn main() {
  let int i = -1;
  while (i != -1) {
    ret;
  }
}
$ ./rustboot -L . ~/tmp/test.rs -o ~/tmp/test
~/tmp/test.rs:3:2 - 6:0:E:Error: Unsatisfied precondition constraint <init #14 = .t0> at stmt 23:
while ({
           auto .t0;
           .t0 = -1;
    }i != .t0) {
    ret;
}
$
```

Interestingly, changing the literal to "1" avoids the error.
