RFR: C1 xchg handling is broken: no item available after WB

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/c1-fix-atomic/webrev.01/

SwingSet2 is the clear reproducer for the problem:

# A fatal error has been detected by the Java Runtime Environment:
#
#  Internal Error
(/home/shade/trunks/shenandoah-jdk/src/hotspot/share/gc/shared/c1/barrierSetC1.hpp:49), pid=69797,
tid=69906
#  assert(_item != __null) failed: sanity
#

This happens because of the simple overlook: we should use _resolved, like any other post-WB user
does. I have checked SBSC1::atomic_xchg_at is the only broken place like that.

Testing: tier3_gc_shenandoah, SwingSet2

Thanks,
-Aleksey