Use the macro assembler Set instead of explicit xor for clearing registers.

src/x64/debug-x64.cc

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions 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 62 matching lines @@
       __ push(reg);
       __ sar(kScratchRegister, Immediate(32));
       __ Integer32ToSmi(kScratchRegister, kScratchRegister);
       __ push(kScratchRegister);
     }
   }
 
 #ifdef DEBUG
   __ RecordComment("// Calling from debug break to runtime - come in - over");
 #endif
-  __ xor_(rax, rax);  // No arguments (argc == 0).
+  __ Set(rax, 0);  // No arguments (argc == 0).
   __ movq(rbx, ExternalReference::debug_break());
 
   CEntryStub ceb(1);
   __ CallStub(&ceb);
 
   // Restore the register values from the expression stack.
   for (int i = kNumJSCallerSaved - 1; i >= 0; i--) {
     int r = JSCallerSavedCode(i);
     Register reg = { r };
     if (FLAG_debug_code) {
@@ skipping 209 matching lines @@
   ASSERT(IsDebugBreakSlot());
   rinfo()->PatchCode(original_rinfo()->pc(), Assembler::kDebugBreakSlotLength);
 }
 
 
 #endif  // ENABLE_DEBUGGER_SUPPORT
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64