[date] Refactor the %_DateField intrinsic to be optimizable.

src/runtime/runtime-date.cc

Index: src/runtime/runtime-date.cc
diff --git a/src/runtime/runtime-date.cc b/src/runtime/runtime-date.cc
index d2b93f03684e9db5238865f7a55c4e8e85ca94e2..47342f98b1a341bc8f42292fe11cbf7b373819df 100644
--- a/src/runtime/runtime-date.cc
+++ b/src/runtime/runtime-date.cc
@@ -60,6 +60,19 @@ RUNTIME_FUNCTION(Runtime_DateSetValue) {
 }
 
 
+RUNTIME_FUNCTION(Runtime_ThrowIfNotADate) {
+  SealHandleScope shs(isolate);
+  DCHECK_EQ(1, args.length());
+  CONVERT_ARG_CHECKED(Object, obj, 0);
+  if (!obj->IsJSDate()) {
+    HandleScope scope(isolate);
+    THROW_NEW_ERROR_RETURN_FAILURE(
+        isolate, NewTypeError(MessageTemplate::kNotDateObject));
+  }
+  return obj;
+}
+
+
 RUNTIME_FUNCTION(Runtime_ThrowNotDateError) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 0);
@@ -174,17 +187,13 @@ RUNTIME_FUNCTION(Runtime_DateCacheVersion) {
 
 RUNTIME_FUNCTION(Runtime_DateField) {
   SealHandleScope shs(isolate);
-  DCHECK(args.length() == 2);
-  CONVERT_ARG_CHECKED(Object, obj, 0);
+  DCHECK_EQ(2, args.length());
+  CONVERT_ARG_CHECKED(JSDate, date, 0);
   CONVERT_SMI_ARG_CHECKED(index, 1);
-  if (!obj->IsJSDate()) {
-    HandleScope scope(isolate);
-    THROW_NEW_ERROR_RETURN_FAILURE(
-        isolate, NewTypeError(MessageTemplate::kNotDateObject));
-  }
-  JSDate* date = JSDate::cast(obj);
+  DCHECK_LE(0, index);
   if (index == 0) return date->value();
   return JSDate::GetField(date, Smi::FromInt(index));
 }
+
 }  // namespace internal
 }  // namespace v8