`no-self-assign` rule failing for shorthand assignment operators

``` javascript
/*eslint no-self-assign: 2*/
var a = 5;
a += a;        // error 'a' is assigned to itself
```

This fails for all the [similar operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Assignment_Operators) when it should be allowed.
