RFR [9]: Bulk backport to sh/jdk9

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk9-20171204/webrev.01/

Weekend runs for sh/jdk10 seem fine.

Changes:

rev 13746 : [backport] Assert Shenandoah-specific safepoints instead of generic ones
rev 13747 : [backport] Generic verification is possible only at Shenandoah safepoints
rev 13748 : [backport] C2 should use heapword-sized object math
rev 13749 : [backport] Trim/expand test heap sizes to fit small heaps
rev 13750 : Add missing TestShenandoahWithLogLevel test
rev 13751 : [backport] Report illegal transitions verbosely, and remove some no-op transitions
rev 13752 : [backport] Cleanup and refactor Full GC code
rev 13753 : [backport] Humongous regions should support explicit pinning
rev 13754 : [backport] Eagerly drop CSet state from regions during Full GC
rev 13755 : [backport] Region sampling should lock while gathering region data
rev 13756 : [backport] Full GC should compact humongous regions
rev 13757 : [backport] Rework shared bool/enum flags with proper types and synchronization
----- weekend runs tested up to here -----
rev 13758 : [backport] Account trashed regions from coalesced CM-with-UR
rev 13759 : [backport] ShenandoahVerifyOptoBarriers should not fail with disabled barriers
----- nightlies are testing now up to here -----

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey