Scribble miscompiles lets inside of an implication

Given the annotation: ```/// #if_succeeds cond ==> let x := E0 in E1;``` scribble will produce the following instrumentation:

```
let x_0 := E0;
if ((!cond) || E1) {
...
}
```

This is incorrect, since `E0` will always be evaluated, even when `cond` is false. This is can cause unexpected crashes. Fix it.