RFC/RFR: Cherry-pick 8221735: Shenandoah fails ctw/modules/jdk_management_agent.java with Traversal

[Aleksey Shipilev]

Let's cherry pick the fix for CTW failure in sh/jdk:
  https://bugs.openjdk.java.net/browse/JDK-8221735
  http://hg.openjdk.java.net/jdk/jdk/rev/dfaa9daab43c

It would take a while until it gets to sh/jdk with merges.

-- diff -r 092d783d359e src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp
--- a/src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp  Tue Apr 02 13:22:56 2019 +0200
+++ b/src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp  Tue Apr 02 13:25:17 2019 +0200
@@ -2097,10 +2097,11 @@
 }

 int ShenandoahEnqueueBarrierNode::needed(Node* n) {
   if (n == NULL ||
       n->is_Allocate() ||
+      n->Opcode() == Op_ShenandoahEnqueueBarrier ||
       n->bottom_type() == TypePtr::NULL_PTR ||
       (n->bottom_type()->make_oopptr() != NULL && n->bottom_type()->make_oopptr()->const_oop() !=
NULL)) {
     return NotNeeded;
   }
   if (n->is_Phi() ||

Testing: hotspot_gc_shenandoah, CTW tests

Thanks,
-Aleksey