RFR (s): 8251460: Fix the biased-locking code in ObjectSynchronizer::FastHashCode

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8251460
webrev: http://cr.openjdk.java.net/~dholmes/8251460/webrev/

During the review for JDK-8250606 it was noted that the biased-locking 
section of FastHashCode contained a similar assertion regarding 
execution at a safepoint:

       // Relaxing assertion for bug 6320749.
       assert(Universe::verify_in_progress() ||
              !SafepointSynchronize::is_at_safepoint(),
              "biases should not be seen by VM thread here");

which gives the impression that it is okay to execute this code block at 
a safepoint if verify_in_progress() is true. But if we examine the next 
line of code:

   BiasedLocking::revoke(hobj, JavaThread::current());

we find that at a safepoint:
a) JavaThread::current() will assert because the current thread will be 
the VMThread; and
b) BiasedLocking::revoke itself contains an assertion that it is not 
invoked at a safepoint.

In practice there is no issue with revoking the bias at a safepoint, we 
just have to use the right code to do so (as done elsewhere in the same 
file):

if (SafepointSynchronize::is_at_safepoint()) {
   BiasedLocking::revoke_at_safepoint(hobj);
} else {
   BiasedLocking::revoke(hobj, self);
}

Thanks,
David