Region of local mutable variables disagrees with llvm backend semantics

Consider the following program:

```
def test3(): Int = {
val f = {
var l = 1;
box { l }
}
f()
}

def main() = println(test3())
```

It should print `1`, which it does. Except when run with `effekt --backend=llvm --no-optimize`, where it prints
`105971318358032`, some garbage value. The local mutable variable `l` escapes its region, which in the llvm backend operationally is only until the end of the local compound statement (block).