RFR [10], 2018-08-21, Bulk backports to sh/jdk10

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk10-20180821/webrev.01/

This backports recent work to sh/jdk10:

 [backport] Split write barrier paths for mutator and GC workers
 [backport] Fix bad assertion: can see non-empty task queue during canceled GC cycle
 [backport] clean up obsolete c2 code
 [backport] WB slowpath should assist with evacuation of adjacent objects
 [backport] Report heap region stats in proper units
 [backport] Shenandoah changes to allow enabling -Wreorder
 [backport] Report Traversal status in heap sampling
 [backport] Convert magic value to ShenandoahPacingSurcharge
 [backport] Improve Traversal CSet logging
 [backport] Adaptive CSet selection for Traversal
 [backport] Trivial enhancement to avoid costly deletion array element
 [backport] Restore stricter assertions in c1_LIR_x86.cpp
 [backport] Traversal needs to enqueue evacuated oops in codecahe

Testing: tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey