[turbofan] Introduce CheckString simplified operator.

src/compiler/typer.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/typer.h"
 
 #include "src/base/flags.h"
 #include "src/bootstrapper.h"
 #include "src/compilation-dependencies.h"
 #include "src/compiler/common-operator.h"
@@ skipping 1788 matching lines @@
 Type* Typer::Visitor::TypeCheckBounds(Node* node) {
   // TODO(bmeurer): We could do better here based on the limit.
   return Type::Unsigned31();
 }
 
 Type* Typer::Visitor::TypeCheckNumber(Node* node) {
   Type* arg = Operand(node, 0);
   return Type::Intersect(arg, Type::Number(), zone());
 }
 
+Type* Typer::Visitor::TypeCheckString(Node* node) {
+  Type* arg = Operand(node, 0);
+  return Type::Intersect(arg, Type::String(), zone());
+}
+
 Type* Typer::Visitor::TypeCheckIf(Node* node) {
   UNREACHABLE();
   return nullptr;
 }
 
 Type* Typer::Visitor::TypeCheckTaggedPointer(Node* node) {
   Type* arg = Operand(node, 0);
   return Type::Intersect(arg, Type::TaggedPointer(), zone());
 }
 
@@ skipping 785 matching lines @@
   }
   if (Type::IsInteger(*value)) {
     return Type::Range(value->Number(), value->Number(), zone());
   }
   return Type::Constant(value, zone());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8