[tre] redundant local var and/or assignment for TRE-optimized local functions

```haxe
class Main {
static function main() {
function f1() {} // no local var even
f1();

function f2() {} // stored in an immediately initialized local var
f2();
f2();

function ftre() { // separate null-initialized local var + assignment
ftre();
}
ftre();
}
}
```
```js
Main.main = function() {
(function() {
})();
var f2 = function() {
};
f2();
f2();
var ftre = null;
ftre = function() {
while(true) continue;
};
ftre();
};
```

This is of course minor and happens because TRE runs long after the local var is introduced (in the typer?), but still maybe we could clean this up somehow.