Use a special EnumLength field to indicate number of valid enum cache values.

src/x64/lithium-x64.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 78 matching lines @@
   V(ConstantT)                                  \
   V(Context)                                    \
   V(DeclareGlobals)                             \
   V(DeleteProperty)                             \
   V(Deoptimize)                                 \
   V(DivI)                                       \
   V(DoubleToI)                                  \
   V(ElementsKind)                               \
   V(FastLiteral)                                \
   V(FixedArrayBaseLength)                       \
+  V(MapEnumLength)                              \
   V(FunctionLiteral)                            \
   V(GetCachedArrayIndex)                        \
   V(GlobalObject)                               \
   V(GlobalReceiver)                             \
   V(Goto)                                       \
   V(HasCachedArrayIndexAndBranch)               \
   V(HasInstanceTypeAndBranch)                   \
   V(In)                                         \
   V(InstanceOf)                                 \
   V(InstanceOfKnownGlobal)                      \
@@ skipping 886 matching lines @@
   explicit LFixedArrayBaseLength(LOperand* value) {
     inputs_[0] = value;
   }
 
   DECLARE_CONCRETE_INSTRUCTION(FixedArrayBaseLength,
                                "fixed-array-base-length")
   DECLARE_HYDROGEN_ACCESSOR(FixedArrayBaseLength)
 };
 
 
+class LMapEnumLength: public LTemplateInstruction<1, 1, 0> {
+ public:
+  explicit LMapEnumLength(LOperand* value) {
+    inputs_[0] = value;
+  }
+
+  DECLARE_CONCRETE_INSTRUCTION(MapEnumLength, "map-enum-length")
+};
+
+
 class LElementsKind: public LTemplateInstruction<1, 1, 0> {
  public:
   explicit LElementsKind(LOperand* value) {
     inputs_[0] = value;
   }
 
   DECLARE_CONCRETE_INSTRUCTION(ElementsKind, "elements-kind")
   DECLARE_HYDROGEN_ACCESSOR(ElementsKind)
 };
 
@@ skipping 1372 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(LChunkBuilder);
 };
 
 #undef DECLARE_HYDROGEN_ACCESSOR
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::int
 
 #endif  // V8_X64_LITHIUM_X64_H_