Add Object::IsNullOrUndefined(Isolate*) helper method

src/full-codegen/x64/full-codegen-x64.cc

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index a19a68c2e88aec2f279d89c0e37716f0d2eb920d..b2f16f0246671b06323b821fa61e9bc286e2e454 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -496,10 +496,8 @@ void FullCodeGenerator::TestContext::Plug(Handle<Object> lit) const {
                                           true,
                                           true_label_,
                                           false_label_);
-  DCHECK(lit->IsNull(isolate()) || lit->IsUndefined(isolate()) ||
-         !lit->IsUndetectable());
-  if (lit->IsUndefined(isolate()) || lit->IsNull(isolate()) ||
-      lit->IsFalse(isolate())) {
+  DCHECK(lit->IsNullOrUndefined(isolate()) || !lit->IsUndetectable());
+  if (lit->IsNullOrUndefined(isolate()) || lit->IsFalse(isolate())) {
     if (false_label_ != fall_through_) __ jmp(false_label_);
   } else if (lit->IsTrue(isolate()) || lit->IsJSObject()) {
     if (true_label_ != fall_through_) __ jmp(true_label_);