Fix representation inference for mutable double boxes.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 05b31162af5730e4b83bb5844ed1aad40615f823..196f8f0b51d5ee400ef58582f2aecc82f340fa5b 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -714,6 +714,7 @@ class HValue : public ZoneObject {
     if (r.IsTagged()) {
       HType t = type();
       if (t.IsSmi()) return Representation::Smi();
+      // TODO(mstarzinger): This is not correct for mutable HeapNumbers.
       if (t.IsHeapNumber()) return Representation::Double();
       if (t.IsHeapObject()) return r;
       return Representation::None();
@@ -5503,6 +5504,10 @@ class HAllocate V8_FINAL : public HTemplateInstruction<2> {
       return Representation::Integer32();
     }
   }
+  // TODO(mstarzinger): Workaround until we track mutable HeapNumber types.
+  virtual Representation KnownOptimalRepresentation() V8_OVERRIDE {
+    return representation();
+  }
 
   virtual Handle<Map> GetMonomorphicJSObjectMap() {
     return known_initial_map_;