[stubs] Add String and JSReceiver instance type predicates

src/builtins/builtins-object.cc

Index: src/builtins/builtins-object.cc
diff --git a/src/builtins/builtins-object.cc b/src/builtins/builtins-object.cc
index 918eb7ef5c4119518449137a2beff7d458afc4b4..bec4e1244e5181e99a792b8761d0bf214a7004a0 100644
--- a/src/builtins/builtins-object.cc
+++ b/src/builtins/builtins-object.cc
@@ -230,10 +230,8 @@ void IsString(CodeStubAssembler* assembler, compiler::Node* object,
   {
     Node* instance_type = assembler->LoadInstanceType(object);
 
-    assembler->Branch(
-        assembler->Int32LessThan(
-            instance_type, assembler->Int32Constant(FIRST_NONSTRING_TYPE)),
-        if_string, if_notstring);
+    assembler->Branch(assembler->IsStringInstanceType(instance_type), if_string,
+                      if_notstring);
   }
 }
 
@@ -259,10 +257,8 @@ void ReturnIfPrimitive(CodeStubAssembler* assembler,
                        CodeStubAssembler::Label* return_string,
                        CodeStubAssembler::Label* return_boolean,
                        CodeStubAssembler::Label* return_number) {
-  assembler->GotoIf(
-      assembler->Int32LessThan(instance_type,
-                               assembler->Int32Constant(FIRST_NONSTRING_TYPE)),
-      return_string);
+  assembler->GotoIf(assembler->IsStringInstanceType(instance_type),
+                    return_string);
 
   assembler->GotoIf(assembler->Word32Equal(
                         instance_type, assembler->Int32Constant(ODDBALL_TYPE)),