RFR: Fix safepoint

[Roman Kennke]

In Safepoint::push_monitor() we skip over shenandoah barriers when
linking a lock's object to the safepoint. I think that's wrong. The
inputs to a SafepointNode are used to build the oopmap for that
safepoint. If have this wb->sp->lock, and at the safepoint, the output
of the wb is not in the oopmap, we may get a fromspace-ref in the
locking code.

The patch reverts back to the original code that does not skip the
shenandoah barrier. It fixes SPECjvm benchmarks that were crashing
before.

http://cr.openjdk.java.net/~rkennke/fixsafepoint/webrev.01/

Ok to commit+push?

Roman