[analyzer] const propagation for long long string

I think it doesn't seem very good.

```haxe
class Main{
static function main() {
var s = "long long string ssssssssssssssssss.....";
trace(s.charAt(0));
trace(s.charAt(1));
}
}
```

Generated with `-D analyzer-optimize`

```js
Main.main = function() {
console.log("long long string ssssssssssssssssss.....".charAt(0));
console.log("long long string ssssssssssssssssss.....".charAt(1));
};
```