RFR [8], 2018-08-21, Bulk backports to sh/jdk8u

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180821/webrev.01/

This backports recent work to sh/jdk8u:

 [backport] Split write barrier paths for mutator and GC workers
 [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] Convert magic value to ShenandoahPacingSurcharge
 [backport] Trivial enhancement to avoid costly deletion array element

Testing: hotspot_tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey