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

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index e744d53f2ddda747f728e44d99305c22943d7138..0e15a947adf97bd3d58ca09404767d046b5b8e62 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/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);
   }
 }