X64: Make SmiCompare use the Smi-register when possible.

Review URL: http://codereview.chromium.org/3104007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5245 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/x64/macro-assembler-x64.cc

@@ -782,8 +782,8 @@ void MacroAssembler::SmiCompare(Register dst, Smi* src) {
   if (src->value() == 0) {
     testq(dst, dst);
   } else {
-    Move(kScratchRegister, src);
-    cmpq(dst, kScratchRegister);
+    Register constant_reg = GetSmiConstant(src);
+    cmpq(dst, constant_reg);
   }
 }