incorrect stack reference cleanup

[gewang]

class Foo
{
    5 => int x;
}

class Bar
{
    Foo foo;
    fun Foo getFoo() { return foo; }
}

Bar bar;
// prints 1
<<< bar.foo >>>;
// possible bug
bar.getFoo() @=> Foo @ foo; // the presence of @=> leads to a missing Release_Object instruction
// prints 3, should be 2
<<< bar.foo >>>;

[gewang]

addressed in 35ad803 as part of dangling object references tracking and cleanup