[stubs] Add String and JSReceiver instance type predicates

src/builtins/builtins-conversion.cc

Index: src/builtins/builtins-conversion.cc
diff --git a/src/builtins/builtins-conversion.cc b/src/builtins/builtins-conversion.cc
index eccec6b8b9e01b572926324c385fd87372055224..7fbe4f859e64b05903f9ab93e43bfced543b3d41 100644
--- a/src/builtins/builtins-conversion.cc
+++ b/src/builtins/builtins-conversion.cc
@@ -168,10 +168,8 @@ void Builtins::Generate_ToString(CodeStubAssembler* assembler) {
   Node* input_instance_type = assembler->LoadMapInstanceType(input_map);
 
   Label not_string(assembler);
-  assembler->GotoIf(
-      assembler->Int32GreaterThanOrEqual(
-          input_instance_type, assembler->Int32Constant(FIRST_NONSTRING_TYPE)),
-      &not_string);
+  assembler->GotoUnless(assembler->IsStringInstanceType(input_instance_type),
+                        &not_string);
   assembler->Return(input);
 
   Label not_heap_number(assembler);