[analyzer] dead assignment is kept due to earlier usage

Came across some curious behavior of the analyzer's dead code removal - with `-D analyzer-optimize`, `bar = ""` is removed as expected in this case:

```haxe
class Main {
public static function main() {}

function foo(bar:String) {
// trace(bar);
bar = "";
}
}
```

However, once the trace is commented in, it's kept, even though it's before the assignment.

Mostly I was wondering if `bar = ""` could be marked as unused by diagnostics, but the analyzer catching it is a prerequisite I guess. ;)