Data Flow Optimization doesn't optimize global variables

At this moment, the Data Flow Optimization doesn't optimize global variables at all - all global variables are considered `volatile`:

```
var global = 1;

begin
    global = 2;
    print(global);
end;
```
compiles to:
```
set .global 1
set .global 2
print .global
```

This is a temporary fix of #279 until a better solution becomes available.