[turbofan] Fix a -Wsign-compare error under GCC 4.9.2.

src/compiler/linkage-impl.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_LINKAGE_IMPL_H_
 #define V8_COMPILER_LINKAGE_IMPL_H_
 
 #include "src/code-stubs.h"
 #include "src/compiler/osr.h"
 
@@ skipping 217 matching lines @@
         0,                                        // js_parameter_count
         Operator::kNoProperties,                  // properties
         LinkageTraits::CCalleeSaveRegisters(),    // callee-saved registers
         LinkageTraits::CCalleeSaveFPRegisters(),  // callee-saved fp regs
         CallDescriptor::kNoFlags,                 // flags
         "c-call");
   }
 
   static CallDescriptor* GetInterpreterDispatchDescriptor(
       Zone* zone, const MachineSignature* msig) {
-    DCHECK_EQ(0, msig->parameter_count());
+    DCHECK_EQ(0U, msig->parameter_count());
     LocationSignature::Builder locations(zone, msig->return_count(),
                                          msig->parameter_count());
     AddReturnLocations(&locations);
     LinkageLocation target_loc = LinkageLocation::AnyRegister();
     return new (zone) CallDescriptor(          // --
         CallDescriptor::kInterpreterDispatch,  // kind
         kMachNone,                             // target MachineType
         target_loc,                            // target location
         msig,                                  // machine_sig
         locations.Build(),                     // location_sig
@@ skipping 66 matching lines @@
     int parameter_index = 1 + index;  // skip index 0, which is the target.
     return incoming_->GetInputLocation(parameter_index);
   }
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_LINKAGE_IMPL_H_