Negated IIFE in if body produces superfluous !

Test case (that currently fails):

``` js
regression_negate_iife_in_if: {
    options = {
        negate_iife: true,
        conditionals: true,
    }
    input: {
        if (!x) {
            (function () {
                x = {};
            })();
        }
    }
    expect: {
        x || function () {
            x = {};
        }();
    }
}
```
