RFR [11]: 2018-10-29, Bulk backports to sh/jdk11

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20181029/webrev.01/

Small batch of outstanding work from last week:

 [backport] revert some useless C2 changes
 [backport] JVMTI lacks a few GC barriers/hooks
 [backport] Cherry-pick JDK-8212673, fix for Node::eqv_uncast
 [backport] Clean up ShHeap::heap_region_iterate uses
 [backport] Parallel heap region iteration
 [backport] Parallelise "Clear Liveness"
 [backport] Parallelise "Complete Liveness"

Testing: tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey