[turbofan] Use appropriate type for NodeId.

test/unittests/compiler/graph-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 "test/unittests/compiler/graph-unittest.h"
 
 #include "src/compiler/node-properties.h"
 #include "test/unittests/compiler/node-test-utils.h"
 
 namespace v8 {
@@ skipping 118 matching lines @@
 const Operator kDummyOperator(0, Operator::kNoProperties, "Dummy", 0, 0, 0, 1,
                               0, 0);
 
 }  // namespace
 
 
 TEST_F(GraphTest, NewNode) {
   Node* n0 = graph()->NewNode(&kDummyOperator);
   Node* n1 = graph()->NewNode(&kDummyOperator);
   EXPECT_NE(n0, n1);
-  EXPECT_LT(0, n0->id());
-  EXPECT_LT(0, n1->id());
+  EXPECT_LT(0u, n0->id());
+  EXPECT_LT(0u, n1->id());
   EXPECT_NE(n0->id(), n1->id());
   EXPECT_EQ(&kDummyOperator, n0->op());
   EXPECT_EQ(&kDummyOperator, n1->op());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8