MIPS64: Enable shorten-64-to-32 warning.

test/cctest/test-code-stubs-mips64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Rrdistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Rrdistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Rrdistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 126 matching lines @@
 
 static Isolate* GetIsolateFrom(LocalContext* context) {
   return reinterpret_cast<Isolate*>((*context)->GetIsolate());
 }
 
 
 int32_t RunGeneratedCodeCallWrapper(ConvertDToIFunc func,
                                     double from) {
 #ifdef USE_SIMULATOR
   Simulator::current(Isolate::Current())->CallFP(FUNCTION_ADDR(func), from, 0.);
-  return Simulator::current(Isolate::Current())->get_register(v0.code());
+  return static_cast<int32_t>(
+      Simulator::current(Isolate::Current())->get_register(v0.code()));
 #else
   return (*func)(from);
 #endif
 }
 
 
 TEST(ConvertDToI) {
   CcTest::InitializeVM();
   LocalContext context;
   Isolate* isolate = GetIsolateFrom(&context);
@@ skipping 21 matching lines @@
                                         false));
       RunAllTruncationTests(
           RunGeneratedCodeCallWrapper,
           MakeConvertDToIFuncTrampoline(isolate,
                                         source_registers[s],
                                         dest_registers[d],
                                         true));
     }
   }
 }