RFR [11] 2018-11-20: Bulk backports to sh/jdk11

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20181120/webrev.01/

This backports recent work to sh/jdk11. I skipped some C2 patches that are irrelevant in 11, because
GC API is different. This also backports up to last unproblematic change, as far as Gotland sees it.
List of changes:

 [backport] undo useless ifnode.cpp change
 [backport] ShenandoahMemoryPool should support usage threshold
 [backport] Shenandoah filter for gc/TestFullGCCount.java test
 [backport] Move save/restore vector registers into ShenandoahBarrierSetAssembler
 [backport] Compact heuristics adjustments
 [backport] Cherry-pick bulk MarkBitMap clearing methods
 [backport] Shenandoah filter for vmTestbase/nsk/jvmti/scenarios/general_functions/GF08/gf08t001
 [backport] is_gc_barrier_node should accept barriers called with both CallLeaf and CallLeafNoFP
 [backport] Remove ShenandoahWriteBarrierCsetTestInIR flag
 [backport] Trivial C2 cleanups

Testing: tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey