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

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 24b991e831a4102c9dffd412c4390658d6a3c26a..3131651267990ee5e5f201ae602c699307e39726 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1512,6 +1512,12 @@ LInstruction* LChunkBuilder::DoFixedArrayBaseLength(
 }
 
 
+LInstruction* LChunkBuilder::DoMapEnumLength(HMapEnumLength* instr) {
+  LOperand* map = UseRegisterAtStart(instr->value());
+  return DefineAsRegister(new(zone()) LMapEnumLength(map));
+}
+
+
 LInstruction* LChunkBuilder::DoElementsKind(HElementsKind* instr) {
   LOperand* object = UseRegisterAtStart(instr->value());
   return DefineAsRegister(new(zone()) LElementsKind(object));