Optimize the typeof operator.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index a5c4c7c62373c5bad72c17d3c7aed8d6beb1400a..eb8274bfd89bc46d84ef1f6793ecf7be86cd2879 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -5549,9 +5549,16 @@ void LCodeGen::DoFunctionLiteral(LFunctionLiteral* instr) {
 
 void LCodeGen::DoTypeof(LTypeof* instr) {
   DCHECK(ToRegister(instr->context()).is(rsi));
-  LOperand* input = instr->value();
-  EmitPushTaggedOperand(input);
-  CallRuntime(Runtime::kTypeof, 1, instr);
+  DCHECK(ToRegister(instr->value()).is(rbx));
+  Label end, do_call;
+  Register value_register = ToRegister(instr->value());
+  __ JumpIfNotSmi(value_register, &do_call);
+  __ Move(rax, isolate()->factory()->number_string());
+  __ jmp(&end);
+  __ bind(&do_call);
+  TypeofStub stub(isolate());
+  CallCode(stub.GetCode(), RelocInfo::CODE_TARGET, instr);
+  __ bind(&end);
 }