Don't try to unlink instructions twice during GVN

BUG=chrome:175141

Review URL: https://codereview.chromium.org/12211128

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13648 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen.cc

@@ -2720,7 +2720,8 @@ void HGlobalValueNumberer::AnalyzeGraph() {
           map->Add(instr, zone());
         }
       }
-      if (instr->CheckFlag(HValue::kTrackSideEffectDominators)) {
+      if (instr->IsLinked() &&
+          instr->CheckFlag(HValue::kTrackSideEffectDominators)) {
         for (int i = 0; i < kNumberOfTrackedSideEffects; i++) {
           HValue* other = dominators->at(i);
           GVNFlag changes_flag = HValue::ChangesFlagFromInt(i);