Heads-up: barriers for constants crash C2

[Aleksey Shipilev]

Hey Roland,

I'm looking into enabling barriers for constants, because that saves a lot of
final mark time. It seems like we have a C2 bug with barriers around constants.
Change this:

-  experimental(bool, ShenandoahNoBarriersForConst, true,                    \
+  experimental(bool, ShenandoahNoBarriersForConst, false,                   \

...and then run fastdebug with hotspot_gc_shenandoah. Then you will see
TestGCOldWithShenandoah crashing with:

#  Internal Error
(/home/shade/trunks/shenandoah-jdk9/hotspot/src/share/vm/opto/gcm.cpp:749),
pid=12758, tid=12778
#  assert(store->find_edge(load) != -1) failed: missing precedence edge


Could you take a look and squash this thing down?

Thanks,
-Aleksey