Dead If Statement Elimination

I enhanced DCE optimize command to eliminate dead conditional statements. This patch may be useful for implementing issue #93.

```
class _Main {
	static function main(args : string[]) : void {
		if (true) {
			log 'Hello';
			log 'JSX';
		}

		if (false) {
			log 'blah blah';
			log 'blah blah';
		}

		if (false) {
			log 'JSX';
		} else {
			log 'World';
		}
	}
}
```
compiled to
```
/**
 * @param {Array.<undefined|!string>} args
 */
_Main.main$AS = function (args) {
	console.log('Hello');
	console.log('JSX');
	console.log('World');
};
```

For now, elimination of while/do-while/for statements which are dead is not supported.