[turbofan] Fix -Wsign-compare warnings.

src/compiler/simd-scalar-lowering.h

 // Copyright 2016 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_SIMD_SCALAR_LOWERING_H_
 #define V8_COMPILER_SIMD_SCALAR_LOWERING_H_
 
 #include "src/compiler/common-operator.h"
 #include "src/compiler/graph.h"
 #include "src/compiler/machine-operator.h"
@@ skipping 12 matching lines @@
 
   void LowerGraph();
 
   int GetParameterCountAfterLowering();
 
  private:
   enum class State : uint8_t { kUnvisited, kOnStack, kVisited };
 
   enum class SimdType : uint8_t { kInt32, kFloat32 };
 
-  static const size_t kMaxLanes = 4;
+  static const int kMaxLanes = 4;
 
   struct Replacement {
     Node* node[kMaxLanes];
     SimdType type;  // represents what input type is expected
   };
 
   Zone* zone() const { return zone_; }
   Graph* graph() const { return graph_; }
   MachineOperatorBuilder* machine() const { return machine_; }
   CommonOperatorBuilder* common() const { return common_; }
@@ skipping 25 matching lines @@
   Signature<MachineRepresentation>* signature_;
   Node* placeholder_;
   int parameter_count_after_lowering_;
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_SIMD_SCALAR_LOWERING_H_