[turbofan] Introduce ObjectIsString operator.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index 515622e1b80645fbd55905e5c5895f8e284708bd..2ac137641d54cab8605cb1ef47b713c46acc4257 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -250,6 +250,7 @@ class Typer::Visitor : public Reducer {
   static Type* ObjectIsNumber(Type*, Typer*);
   static Type* ObjectIsReceiver(Type*, Typer*);
   static Type* ObjectIsSmi(Type*, Typer*);
+  static Type* ObjectIsString(Type*, Typer*);
   static Type* ObjectIsUndetectable(Type*, Typer*);
 
   static Type* JSAddRanger(RangeType*, RangeType*, Typer*);
@@ -577,6 +578,11 @@ Type* Typer::Visitor::ObjectIsSmi(Type* type, Typer* t) {
   return Type::Boolean();
 }
 
+Type* Typer::Visitor::ObjectIsString(Type* type, Typer* t) {
+  if (type->Is(Type::String())) return t->singleton_true_;
+  if (!type->Maybe(Type::String())) return t->singleton_false_;
+  return Type::Boolean();
+}
 
 Type* Typer::Visitor::ObjectIsUndetectable(Type* type, Typer* t) {
   if (type->Is(Type::Undetectable())) return t->singleton_true_;
@@ -1997,6 +2003,9 @@ Type* Typer::Visitor::TypeObjectIsSmi(Node* node) {
   return TypeUnaryOp(node, ObjectIsSmi);
 }
 
+Type* Typer::Visitor::TypeObjectIsString(Node* node) {
+  return TypeUnaryOp(node, ObjectIsString);
+}
 
 Type* Typer::Visitor::TypeObjectIsUndetectable(Node* node) {
   return TypeUnaryOp(node, ObjectIsUndetectable);