[stubs] Add String and JSReceiver instance type predicates

src/code-stub-assembler.h

Index: src/code-stub-assembler.h
diff --git a/src/code-stub-assembler.h b/src/code-stub-assembler.h
index ec84b38fd73ee4e081ee81842c928fd455e2f760..095b271fadebbbcfc3b3a07f8caef0ac46b67dc4 100644
--- a/src/code-stub-assembler.h
+++ b/src/code-stub-assembler.h
@@ -487,6 +487,10 @@ class CodeStubAssembler : public compiler::CodeAssembler {
                                          InstanceType instance_type,
                                          char const* method_name);
 
+  // Type checks.
+  compiler::Node* IsStringInstanceType(compiler::Node* instance_type);
+  compiler::Node* IsJSReceiverInstanceType(compiler::Node* instance_type);
+
   // String helpers.
   // Load a character from a String (might flatten a ConsString).
   compiler::Node* StringCharCodeAt(compiler::Node* string,