[turbofan] Utilize the fact that empty string is canonicalized.

test/unittests/compiler/js-typed-lowering-unittest.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/js-typed-lowering.h"
 #include "src/code-factory.h"
 #include "src/compilation-dependencies.h"
 #include "src/compiler/access-builder.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/js-operator.h"
@@ skipping 112 matching lines @@
 
 TEST_F(JSTypedLoweringTest, JSToBooleanWithReceiverOrNullOrUndefined) {
   Node* input = Parameter(Type::ReceiverOrNullOrUndefined(), 0);
   Node* context = Parameter(Type::Any(), 1);
   Reduction r = Reduce(graph()->NewNode(
       javascript()->ToBoolean(ToBooleanHint::kAny), input, context));
   ASSERT_TRUE(r.Changed());
   EXPECT_THAT(r.replacement(), IsBooleanNot(IsObjectIsUndetectable(input)));
 }
 
+TEST_F(JSTypedLoweringTest, JSToBooleanWithString) {
+  Node* input = Parameter(Type::String(), 0);
+  Node* context = Parameter(Type::Any(), 1);
+  Reduction r = Reduce(graph()->NewNode(
+      javascript()->ToBoolean(ToBooleanHint::kAny), input, context));
+  ASSERT_TRUE(r.Changed());
+  EXPECT_THAT(r.replacement(),
+              IsBooleanNot(IsReferenceEqual(
+                  input, IsHeapConstant(factory()->empty_string()))));
+}
+
 TEST_F(JSTypedLoweringTest, JSToBooleanWithAny) {
   Node* input = Parameter(Type::Any(), 0);
   Node* context = Parameter(Type::Any(), 1);
   Reduction r = Reduce(graph()->NewNode(
       javascript()->ToBoolean(ToBooleanHint::kAny), input, context));
   ASSERT_FALSE(r.Changed());
 }
 
 
 // -----------------------------------------------------------------------------
@@ skipping 864 matching lines @@
                                         EmptyFrameState(), effect, control));
   ASSERT_TRUE(r.Changed());
   EXPECT_THAT(r.replacement(), IsSpeculativeNumberBitwiseXor(
                                    NumberOperationHint::kNumberOrOddball, lhs,
                                    rhs, effect, control));
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8