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

src/x64/lithium-x64.h

Index: src/x64/lithium-x64.h
diff --git a/src/x64/lithium-x64.h b/src/x64/lithium-x64.h
index a7ae588ff9a90420a690efac2a4a32dc80522d87..d6c6a5d7bb93d26d8015f0eadb9ff8b19cdd4b0c 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -96,6 +96,7 @@ class LCodeGen;
   V(ElementsKind)                               \
   V(FastLiteral)                                \
   V(FixedArrayBaseLength)                       \
+  V(MapEnumLength)                              \
   V(FunctionLiteral)                            \
   V(GetCachedArrayIndex)                        \
   V(GlobalObject)                               \
@@ -994,6 +995,18 @@ class LFixedArrayBaseLength: public LTemplateInstruction<1, 1, 0> {
 };
 
 
+class LMapEnumLength: public LTemplateInstruction<1, 1, 0> {
+ public:
+  explicit LMapEnumLength(LOperand* value) {
+    inputs_[0] = value;
+  }
+
+  DECLARE_CONCRETE_INSTRUCTION(MapEnumLength,
+                               "map-enum-length")
+  DECLARE_HYDROGEN_ACCESSOR(MapEnumLength)
+};
+
+
 class LElementsKind: public LTemplateInstruction<1, 1, 0> {
  public:
   explicit LElementsKind(LOperand* value) {