RFR: Double check for UseShenandoahGC in WB expand

[Aleksey Shipilev]

Trivial thing found by adinn during 8u integration review. Fix this in sh/jdk10, and let is sink
through to sh/jdk8u later:

diff -r 8dbba95d82ba src/share/vm/opto/shenandoahSupport.cpp
--- a/src/share/vm/opto/shenandoahSupport.cpp	Mon Oct 02 13:16:10 2017 +0200
+++ b/src/share/vm/opto/shenandoahSupport.cpp	Wed Oct 04 13:05:23 2017 +0200
@@ -536,18 +536,14 @@
 }

 bool ShenandoahWriteBarrierNode::expand(Compile* C, PhaseIterGVN& igvn, int& loop_opts_cnt) {
-  if (ShenandoahWriteBarrierToIR) {
+  if (UseShenandoahGC && ShenandoahWriteBarrierToIR) {
     if (C->shenandoah_barriers_count() > 0) {
       bool attempt_more_loopopts = ShenandoahLoopOptsAfterExpansion &&
(C->shenandoah_barriers_count() > 1 || C->has_loops());
       C->clear_major_progress();
       PhaseIdealLoop ideal_loop(igvn, LoopOptsShenandoahExpand);
       if (C->failing()) return false;
       PhaseIdealLoop::verify(igvn);
-#ifdef ASSERT
-      if (UseShenandoahGC) {
-        ShenandoahBarrierNode::verify_raw_mem(C->root());
-      }
-#endif
+      DEBUG_ONLY(ShenandoahBarrierNode::verify_raw_mem(C->root());)
       if (attempt_more_loopopts) {
         C->set_major_progress();
         if (!C->optimize_loops(loop_opts_cnt, igvn, LoopOptsShenandoahPostExpand)) {

Testing: hotspot_fast_gc_shenandoah

Thanks,
-Aleksey