Define reference reachability more precisely in java.lang.ref package

Phantom referenced objects may not appear in the ReferenceQueue if VM run with -Xcomp.

The minimal test demonstrating the bug looks like:
--- 8< ------- Phantom.java --------------------
import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;

public class Phantom {
    static ReferenceQueue rq = new ReferenceQueue();
    public static void main(final String[] args) throws Exception {
        PhantomReference ref = new PhantomReference(new Object(), rq);
        System.out.println("ref: " + ref);
        System.gc();

        Reference rmRef = rq.remove(1000);
        if (rmRef == null) {
            for (int i = 0; i < 10; i++) {
                System.gc();
            }
            rmRef = rq.remove(1000);
            if (rmRef == null) {
                throw new Error("Test failed");
            }
        }
        System.out.println("Test passed");
    }
}
--- 8< ---------------------------------------------------------

# java Phantom
ref: java.lang.ref.PhantomReference@614c5515
Test passed
# java -Xcomp Phantom
ref: java.lang.ref.PhantomReference@614c5515
Exception in thread "main" java.lang.Error: Test failed
        at Phantom.main(Phantom.java:19)

Please note: Declaring 'PhantomReference ref' as a static member of the Phantom class, not as a local variable, will make the test pass.