revoke_bias() needs a new assertion

A comment from Robin W's code review on JDK-8167108:

> src/hotspot/share/runtime/biasedLocking.cpp
>
> 217 for (JavaThread* cur_thread = Threads::first(); cur_thread != NULL; cur_thread = cur_thread->next()) {
> 218 if (cur_thread == biased_thread) {
> 219 thread_is_alive = true;
>
> This loop could be replaced with:
>
> ThreadsListHandle tlh;
> thread_is_alive = tlh.includes(biased_thread);

Nice catch! Fixed with your proposed change.

The careful reader will notice that in revoke_bias() we are not
creating a ThreadsListHandle that is in scope for the entire
function and we are doing cached monitor info walks without an
obvious ThreadsListHandle in place.

revoke_bias() is called at a safepoint from most locations. The
one call that is not made at a safepoint is from
BiasedLocking::revoke_and_rebias() and it is made by a JavaThread
that is revoking a bias on itself which is also safe.

We should add an assertion to revoke_bias() that looks something
like this:

  assert(requesting_thread == Thread::current() ||
         SafepointSynchronize::is_at_safepoint(),
         "must be operating on self or at a safepoint.");

but we'll do that in a separate follow up bug since that will
require biased locking focused testing.