Avoid re-tagging of the index register in optimized byte array access.

runtime/vm/locations.cc

Index: runtime/vm/locations.cc
===================================================================
--- runtime/vm/locations.cc	(revision 18444)
+++ runtime/vm/locations.cc	(working copy)
@@ -78,6 +78,14 @@
 }
 
 
+Location Location::WritableRegisterOrSmiConstant(Value* value) {
+  ConstantInstr* constant = value->definition()->AsConstant();
+  return ((constant != NULL) && constant->value().IsSmi())
+      ? Location::Constant(constant->value())
+      : Location::WritableRegister();
+}
+
+
 Location Location::AnyOrConstant(Value* value) {
   ConstantInstr* constant = value->definition()->AsConstant();
   return (constant != NULL)