Add LoadTaggedClassIdMayBeSmi to assembly and improve performance of inline cache stubs.

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
===================================================================
--- runtime/vm/intermediate_language_x64.cc	(revision 36956)
+++ runtime/vm/intermediate_language_x64.cc	(working copy)
@@ -868,15 +868,7 @@
 void LoadClassIdInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   Register object = locs()->in(0).reg();
   Register result = locs()->out(0).reg();
-  Label load, done;
-  __ testq(object, Immediate(kSmiTagMask));
-  __ j(NOT_ZERO, &load, Assembler::kNearJump);
-  __ LoadImmediate(result, Immediate(Smi::RawValue(kSmiCid)), PP);
-  __ jmp(&done);
-  __ Bind(&load);
-  __ LoadClassId(result, object);
-  __ SmiTag(result);
-  __ Bind(&done);
+  __ LoadTaggedClassIdMayBeSmi(result, object);
 }