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

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 68795a48c1f4e6ca73c282cd8983d9ae290cb775..b0c65e26cdcc669f4e817fdc16de9332c25aef61 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -945,6 +945,15 @@ class MacroAssembler: public Assembler {
                           Register temp_reg);
 
   void LoadInstanceDescriptors(Register map, Register descriptors);
+  void EnumLength(Register dst, Register map);
+
+  template<typename Field>
+  void DecodeField(Register reg) {
+    static const int full_shift = Field::kShift + kSmiShift;
+    static const int low_mask = Field::kMask >> Field::kShift;
+    shr(reg, Immediate(full_shift));
+    and_(reg, Immediate(low_mask));
+  }
 
   // Abort execution if argument is not a number. Used in debug code.
   void AbortIfNotNumber(Register object);