promote complex right-hand exprs for OpAssignOp as well

Subj.

Example code:
```haxe
class Main {
    static function main() {
        function f() { return 0; }

        var time = 0;
        time += Std.int(f());
    }
}
```

Before (JS):
```js
Main.main = function() {
	var f = function() {
		return 0;
	};
	var time = 0;
	time += (function($this) {
		var $r;
		var x = f();
		$r = x | 0;
		return $r;
	}(this));
};
```

Before (After):
```js
Main.main = function() {
	var f = function() {
		return 0;
	};
	var time = 0;
	var x = f();
	time += x | 0;
};
```