Revert of [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 6e23d2913c023d14717a16833ebcbbc9316a2176..f9b3f39ac90e024e799a70f7863e7d79f4ef4adc 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -915,7 +915,6 @@
 
 
 Register MacroAssembler::GetSmiConstant(Smi* source) {
-  STATIC_ASSERT(kSmiTag == 0);
   int value = source->value();
   if (value == 0) {
     xorl(kScratchRegister, kScratchRegister);
@@ -927,13 +926,7 @@
 
 
 void MacroAssembler::LoadSmiConstant(Register dst, Smi* source) {
-  STATIC_ASSERT(kSmiTag == 0);
-  int value = source->value();
-  if (value == 0) {
-    xorl(dst, dst);
-  } else {
-    Move(dst, source, Assembler::RelocInfoNone());
-  }
+  Move(dst, source, Assembler::RelocInfoNone());
 }