C2, pre-barrier, getAndSet

[Aleksey Shipilev]

Here is the failure:

$
~/trunks/shenandoah-jdk9/build/linux-x86_64-normal-server-fastdebug/jdk/bin/java
-XX:+UseShenandoahGC -XX:+UseShenandoahMatrix -XX:+VerifyShenandoahMatrix -jar
tests-all/target/jcstress.jar -t
org.openjdk.jcstress.tests.atomicity.varHandles.fields.GetAndSetTest.GetAndSetString

#  Internal Error
(/home/shade/trunks/shenandoah-jdk9/hotspot/src/share/vm/opto/split_if.cpp:287),
pid=10961, tid=10984
#  assert(prior_n->is_Region()) failed: must be a post-dominating merge point

Patches, hs_errs, replays here:
  http://cr.openjdk.java.net/~shade/shenandoah/wtf-c2-getandset/

Thanks,
-Aleksey