Fix LoadFieldByIndex to take mutable heap-numbers into account.

src/x64/lithium-codegen-x64.h

Index: src/x64/lithium-codegen-x64.h
diff --git a/src/x64/lithium-codegen-x64.h b/src/x64/lithium-codegen-x64.h
index 37807ede0dfa094c1446a4264ac8b1996d622fca..23c63c8160628384190c87ed5805e1357319325f 100644
--- a/src/x64/lithium-codegen-x64.h
+++ b/src/x64/lithium-codegen-x64.h
@@ -116,6 +116,9 @@ class LCodeGen: public LCodeGenBase {
   void DoDeferredInstanceOfKnownGlobal(LInstanceOfKnownGlobal* instr,
                                        Label* map_check);
   void DoDeferredInstanceMigration(LCheckMaps* instr, Register object);
+  void DoDeferredLoadMutableDouble(LLoadFieldByIndex* instr,
+                                   Register object,
+                                   Register index);
 
 // Parallel move support.
   void DoParallelMove(LParallelMove* move);