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

src/compiler/typer.h

 // 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.
 
 #ifndef V8_COMPILER_TYPER_H_
 #define V8_COMPILER_TYPER_H_
 
 #include "src/compiler/graph.h"
 #include "src/compiler/operation-typer.h"
 #include "src/globals.h"
@@ skipping 32 matching lines @@
   Isolate* isolate() const { return isolate_; }
   OperationTyper* operation_typer() { return &operation_typer_; }
 
   Isolate* const isolate_;
   Flags const flags_;
   Graph* const graph_;
   Decorator* decorator_;
   TypeCache const& cache_;
   OperationTyper operation_typer_;
 
+  Type* singleton_empty_string_;
   Type* singleton_false_;
   Type* singleton_true_;
-  Type* singleton_the_hole_;
   Type* falsish_;
   Type* truish_;
 
   DISALLOW_COPY_AND_ASSIGN(Typer);
 };
 
 DEFINE_OPERATORS_FOR_FLAGS(Typer::Flags);
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_TYPER_H_