[x64] Use xorl to materialize smi zero.

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 efe90044409ebeeaee9d10b59de3f0a8fc9a9071..88ef9824f8d58c8d7ef53863395f776b3a14161f 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -1100,6 +1100,7 @@ void MacroAssembler::SafePush(Smi* src) {
 
 
 Register MacroAssembler::GetSmiConstant(Smi* source) {
+  STATIC_ASSERT(kSmiTag == 0);
   int value = source->value();
   if (value == 0) {
     xorl(kScratchRegister, kScratchRegister);
@@ -1111,9 +1112,13 @@ Register MacroAssembler::GetSmiConstant(Smi* source) {
 
 
 void MacroAssembler::LoadSmiConstant(Register dst, Smi* source) {
-  // Special-casing 0 here to use xorl seems to make things slower, so we don't
-  // do it.
-  Move(dst, source, Assembler::RelocInfoNone());
+  STATIC_ASSERT(kSmiTag == 0);
+  int value = source->value();
+  if (value == 0) {
+    xorl(dst, dst);
+  } else {
+    Move(dst, source, Assembler::RelocInfoNone());
+  }
 }