New hydrogen instruction to reduce cost of growing an array on keyed stores.

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 0e70826df2297d003f28c3de27577394fd9b57a2..9adf6a45a9cdc66dfad83f780858b5be00ea5eac 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -925,6 +925,8 @@ 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());
 }