Garbage Collector, call & callvirt and Debug/Release code mode execution differences

I have a class:

public class SomeClass {
    public int I;

    public SomeClass(int input) {
        I = input;
        Console.WriteLine("I = {0}", I);
    }

    ~SomeClass() {
        Console.WriteLine("deleted");
    }

    public void Foo() {
        Thread.Sleep(1000);
        Console.WriteLine("Foo");
    }
}

and this program:

class Program {
    static void Main(string[] args) {
        new Thread(() => {
                       Thread.Sleep(100);
                       GC.Collect();
                   }) { IsBackground = true }.Start();

        new SomeClass(10).Foo();

        // The same as upper code
        // var t = new SomeClass(10);
        // t.Foo();
    }
}

When i run this code in Debug mode, i have next result:

I = 10
Foo
deleted

but, when i change mode to Release, result changes to:

I = 10
deleted
Foo

As i understand, there is difference with call and callvirt: when optimization starts in Release mode, compiler look at Foo method and can't find any reference to SomeClass in this method, and that's why this method calls as static method by address, and garbage collector could collect this object.

Otherwise, if i change Foo method (for example, add Console.WriteLine(I) into this method), compiler won't decide to call this method as call and it should be called by pointer to instance with callvirt and garbage collector won't collect this object.

Can you please explain more clearly, what's going on here (why GC could collect object and if it's so, how does method calls).