Fix a bug with an unused variable.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 24909c1e9fa50ce08c114452894d48d9e60b2d34..20362bd71e3b98bfdac05cec95cedfb245b2ce7e 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -1757,6 +1757,10 @@ Definition* CheckedSmiOpInstr::Canonicalize(FlowGraph* flow_graph) {
       default:
         break;
     }
+    if (replacement != NULL) {
+      flow_graph->InsertBefore(this, replacement, env(), FlowGraph::kValue);
+      return replacement;
+    }
   }
   return this;
 }