Fix compilation failure when v8 compiled with GCC 5.x due to `-Werror=unused-variable`

test/unittests/compiler/linkage-tail-call-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 "src/compiler/common-operator.h"
 #include "src/compiler/graph.h"
 #include "src/compiler/linkage.h"
 #include "src/compiler/node.h"
 #include "test/unittests/test-utils.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
 
 namespace {
 
 MachineType kMachineTypes[] = {
     MachineType::AnyTagged(), MachineType::AnyTagged(),
     MachineType::AnyTagged(), MachineType::AnyTagged(),
     MachineType::AnyTagged(), MachineType::AnyTagged(),
     MachineType::AnyTagged(), MachineType::AnyTagged()};
 }
 
 class LinkageTailCall : public TestWithZone {
  protected:
   CallDescriptor* NewStandardCallDescriptor(LocationSignature* locations) {
     DCHECK(arraysize(kMachineTypes) >=
            locations->return_count() + locations->parameter_count());
+    USE(kMachineTypes);
     return new (zone()) CallDescriptor(
         CallDescriptor::kCallCodeObject, MachineType::AnyTagged(),
         LinkageLocation::ForAnyRegister(MachineType::Pointer()),
         locations,                 // location_sig
         0,                         // js_parameter_count
         Operator::kNoProperties,   // properties
         0,                         // callee-saved
         0,                         // callee-saved fp
         CallDescriptor::kNoFlags,  // flags,
         "");
@@ skipping 300 matching lines @@
   Node* const node =
       Node::New(zone(), 1, op, arraysize(parameters), parameters, false);
   EXPECT_TRUE(desc1->CanTailCall(node));
   int stack_param_delta = desc2->GetStackParameterDelta(desc1);
   EXPECT_EQ(1, stack_param_delta);
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8