GC doesn't collect where expected

[axricard]

I'm doing some experiments with ldc2 GC, by instrumenting it and 
printing basic information (what is allocated and freed)

My first tests are made on this sample :

```
>> cat test2.d
import core.memory;

class Bar { int bar; }

class Foo {

   this()
   {
     this.bar = new Bar;
   }

   Bar bar;
}


void func()
{
   Foo f2 = new Foo;
}

int main()
{
   Foo f = new Foo;

   func();
   GC.collect();

   return 0;
}

```

When trying to run the instrumented druntime, I get a strange 
behavior : the first collection (done with GC.collect) doesn't 
sweep anything (in particular, it doesn't sweep memory allocated 
in _func()_). The whole sweeping is done when program finish, at 
cleanup. I don't understand why : memory allocated in _func()_ 
shouldn't be accessible from any root at first collection, right ?

```
╰─> /instrumented-ldc2 -g -O0 test2.d --disable-gc2stack 
--disable-d-passes --of test2  &&  ./test2 
"--DRT-gcopt=cleanup:collect fork:0 parallel:0 verbose:2"


[test2.d:26] new 'test2.Foo' (24 bytes) => p = 0x7f3a0454d000
[test2.d:10] new 'test2.Bar' (20 bytes) => p = 0x7f3a0454d020
[test2.d:21] new 'test2.Foo' (24 bytes) => p = 0x7f3a0454d040
[test2.d:10] new 'test2.Bar' (20 bytes) => p = 0x7f3a0454d060

============ COLLECTION  =============
         ============= MARKING ==============
         marking range: [0x7fff22337a60..0x7fff22339000] (0x15a0)
                 range: [0x7f3a0454d000..0x7f3a0454d020] (0x20)
                 range: [0x7f3a0454d040..0x7f3a0454d060] (0x20)
         marking range: [0x7f3a0464d720..0x7f3a0464d8b9] (0x199)
         marking range: [0x46c610..0x47b3b8] (0xeda8)
         ============= SWEEPING ==============
=====================================================


============ COLLECTION  =============
         ============= MARKING ==============
         marking range: [0x46c610..0x47b3b8] (0xeda8)
         ============= SWEEPING ==============
         Freeing test2.Foo (test2.d:26; 24 bytes) 
(0x7f3a0454d000). AGE :  1/2
         Freeing test2.Bar (test2.d:10; 20 bytes) 
(0x7f3a0454d020). AGE :  1/2
         Freeing test2.Foo (test2.d:21; 24 bytes) 
(0x7f3a0454d040). AGE :  1/2
         Freeing test2.Bar (test2.d:10; 20 bytes) 
(0x7f3a0454d060). AGE :  1/2
=====================================================
```