RFR: [8u] Bulk backports to sh/jdk8u

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20171205/webrev.01/

Changes:

46091fe1a0bc: [backport] Assert Shenandoah-specific safepoints instead of generic ones
2d0fb36d2bb3: [backport] Generic verification is possible only at Shenandoah safepoints
956b6e15ff46: [backport] C2 should use heapword-sized object math
8aa58b6572a3: [backport] Trim/expand test heap sizes to fit small heaps
d8f6b4e791f5: [backport] Report illegal transitions verbosely, and remove some no-op transitions
1e9a0e68a087: [backport] Cleanup and refactor Full GC code
9beca79e01ca: [backport] Humongous regions should support explicit pinning
efd9de15c656: [backport] Eagerly drop CSet state from regions during Full GC
b067065f7bde: [backport] Region sampling should lock while gathering region data
e64c7ea17e9c: [backport] Full GC should compact humongous regions
d54166ac952d: [backport] Rework shared bool/enum flags with proper types and synchronization
c6e7e25780c1: [backport] Account trashed regions from coalesced CM-with-UR
9cafa38b1ef1: [backport] ShenandoahVerifyOptoBarriers should not fail with disabled barriers

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey