RFR: Protect C2 matchers with UseShenandoahGC

[Aleksey Shipilev]

These matchers are used in shared code to act on Shenandoah code. While it works, because they match
the particular TLS+offset that is Shenandoah-specific, it is still fragile. I would prefer us to
chicken out when Shenandoah is disabled:

diff -r 397c505d68f5 src/hotspot/share/opto/shenandoahSupport.cpp
--- a/src/hotspot/share/opto/shenandoahSupport.cpp	Mon Feb 19 16:58:32 2018 +0100
+++ b/src/hotspot/share/opto/shenandoahSupport.cpp	Mon Feb 19 18:15:37 2018 +0100
@@ -557,6 +557,9 @@
 }

 bool ShenandoahWriteBarrierNode::is_evacuation_in_progress_test(Node* iff) {
+  if (!UseShenandoahGC) {
+    return false;
+  }
   assert(iff->is_If(), "bad input");
   if (iff->Opcode() != Op_If) {
     return false;
@@ -587,6 +590,9 @@
 }

 bool ShenandoahWriteBarrierNode::is_gc_state_load(Node *n) {
+  if (!UseShenandoahGC) {
+    return false;
+  }
   if (n->Opcode() != Op_LoadUB && n->Opcode() != Op_LoadB) {
     return false;
   }

Testing: hotspot_fast_gc_shenandoah

Thanks,
-Aleksey