Extract hardcoded error strings into a single place and replace them with enum.

test/cctest/test-code-stubs-x64.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 75 matching lines @@
   // Call through to the actual stub
   __ Call(start, RelocInfo::EXTERNAL_REFERENCE);
 
   __ addq(rsp, Immediate(kDoubleSize));
 
   // Make sure no registers have been unexpectedly clobbered
   for (--reg_num; reg_num >= 0; --reg_num) {
     Register reg = Register::from_code(reg_num);
     if (!reg.is(rsp) && !reg.is(rbp) && !reg.is(destination_reg)) {
       __ cmpq(reg, MemOperand(rsp, 0));
-      __ Assert(equal, "register was clobbered");
+      __ Assert(equal, kRegisterWasClobbered);
       __ addq(rsp, Immediate(kPointerSize));
     }
   }
 
   __ movq(rax, destination_reg);
 
   __ pop(rdi);
   __ pop(rsi);
   __ pop(rdx);
   __ pop(rcx);
@@ skipping 33 matching lines @@
 
   for (size_t s = 0; s < sizeof(*source_registers); s++) {
     for (size_t d = 0; d < sizeof(*dest_registers); d++) {
       RunAllTruncationTests(
           MakeConvertDToIFuncTrampoline(isolate,
                                         source_registers[s],
                                         dest_registers[d]));
     }
   }
 }