RFR: Cleanup code in C2 related to G1/Sh pre-barrier handling

[Roman Kennke]

Another round of clean-ups in C2: this time some code pieces related to
pre-barrier handling.

As far as I know/remember, all of those code in shared path has
originally been duplicated from corresponding G1 code paths. Since GC
interface arrived, the G1 counterparts have since been removed. At least
I can not find where they could have gone in G1BarrierSetC2. I therefore
propose to do the same for the Shenandoah parts, under the assumption
that this would not have ill-effects. Roland: can you carefully check this?

The utility methods in node.hpp to detect certain properties have been
moved to ShenandoahBarrierSetC2, the only uses left are in Shenandoah land.

Tests: tier3_gc_shenandoah

http://cr.openjdk.java.net/~rkennke/cleanup-c2-pre-barrier/webrev.00/

WDYT?
Roman