[turbofan] Improve typed lowering rules for JSToBoolean.

test/unittests/compiler/graph-unittest.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_UNITTESTS_COMPILER_GRAPH_UNITTEST_H_
 #define V8_UNITTESTS_COMPILER_GRAPH_UNITTEST_H_
 
 #include "src/compiler/common-operator.h"
 #include "src/compiler/graph.h"
 #include "src/compiler/typer.h"
@@ skipping 35 matching lines @@
   Node* TrueConstant();
   Node* UndefinedConstant();
 
   Node* EmptyFrameState();
 
   Matcher<Node*> IsBooleanConstant(bool value) {
     return value ? IsTrueConstant() : IsFalseConstant();
   }
   Matcher<Node*> IsFalseConstant();
   Matcher<Node*> IsTrueConstant();
+  Matcher<Node*> IsNullConstant();
   Matcher<Node*> IsUndefinedConstant();
 
   CommonOperatorBuilder* common() { return &common_; }
   Graph* graph() { return &graph_; }
 
  private:
   CommonOperatorBuilder common_;
   Graph graph_;
 };
 
@@ skipping 11 matching lines @@
 
  private:
   Typer typer_;
 };
 
 }  //  namespace compiler
 }  //  namespace internal
 }  //  namespace v8
 
 #endif  // V8_UNITTESTS_COMPILER_GRAPH_UNITTEST_H_