RFR: [8] 2018-06-25: Bulk backports to sh/jdk8

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180625/webrev.01/

This backports recent performance and bugfixing work to sh/jdk8:

 [backport] Move (Java)Thread::_gc_state to lower offset to optimize barrier fast-path encoding
 [backport] SH::make_(tlabs)_parsable() should work correctly with/without TLABs
 [backport] Removed racy assertion
 [backport] AlwaysPreTouch fails with non-default ConcGCThreads
 [backport] Improve scheduling and interleaving of SATB processing in mark loop
 [backport] Apply ShenandoahEvacOOMScope only for evac-taking paths in ShenandoahBarrierSet
 [backport] Replace risky SBS::need_update_refs_barrier with straightforward check
 [backport] Pre-filter oops before enqueing them in SBS slowpaths
 [backport] SATB buffer filtering/compaction hides unmarked objects until final-mark
 [backport] Process remaining SATB buffers in final mark/traverse loop instead of separate phase
 [backport] Skip RESOLVE in SATBBufferClosure if no forwarded objects are in heap
 [backport] VSC++ requires space(s) in between two string literals
 [backport] Missing Shenandoah entry in GCNameHelper::to_string

Testing: hotspot_tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey