fix -Wunused-but-set-variable for gcc-4.6 on x64

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
diff --git a/src/x64/code-stubs-x64.cc b/src/x64/code-stubs-x64.cc
index 3cf7840d54997155496dd9837dcd84f23b5c966c..cae340e7b2743afc37f38daf52464f740f6ce121 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -2496,8 +2496,10 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   Register arg4 = rcx;
   Register arg3 = rdx;
   Register arg2 = rsi;
+#ifdef DEBUG
   Register arg1 = rdi;

[William Hesse, 2011/07/19 12:16:43]

This is so asymmetric, and weird, that I would prefer a USE(arg1); line below,
instead of not declaring it.
I assume that the USE() macro also gets rid of unused variable errors?

[wingo, 2011/07/19 13:52:10]

Ah, I had not seen USE; thanks for the pointer. It's a template actually. 
Fixed.

 #endif
+#endif
 
   // Keep track on aliasing between argX defined above and the registers used.
   // rdi: subject string