Revert of [x64] Use xorl to materialize smi zero. (patchset #1 id:1 of …

…https://codereview.chromium.org/1085153002/)

Reason for revert:
Seems to cause performance regressions.

Original issue's description:
> [x64] Use xorl to materialize smi zero.
>
> Before we always loaded smi zero via a movabs with a 64-bit immediate,
> which is pretty expensive compared to the xorl.
>
> R=jarin@chromium.org
>
> Committed: https://crrev.com/f236777bfe6e080ff1ead6baf847cc9b6bb4f9cb
> Cr-Commit-Position: refs/heads/master@{#27829}

TBR=jarin@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:477592
LOG=n

Review URL: https://codereview.chromium.org/1059543004

Cr-Commit-Position: refs/heads/master@{#27867}

src/x64/macro-assembler-x64.cc

@@ -915,7 +915,6 @@ void MacroAssembler::SafePush(Smi* src) {
 
 
 Register MacroAssembler::GetSmiConstant(Smi* source) {
-  STATIC_ASSERT(kSmiTag == 0);
   int value = source->value();
   if (value == 0) {
     xorl(kScratchRegister, kScratchRegister);
@@ -927,13 +926,7 @@ Register MacroAssembler::GetSmiConstant(Smi* source) {
 
 
 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());
 }