Use SAR for positive divident by a power-of two constant divisor.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
===================================================================
--- runtime/vm/intermediate_language.cc	(revision 17939)
+++ runtime/vm/intermediate_language.cc	(working copy)
@@ -1362,6 +1362,15 @@
 }
 
 
+bool BinarySmiOpInstr::RightIsPowerOfTwoConstant() const {
+  if (!right()->definition()->IsConstant()) return false;
+  const Object& constant = right()->definition()->AsConstant()->value();
+  if (!constant.IsSmi()) return false;
+  const intptr_t int_value = Smi::Cast(constant).Value();
+  return (int_value > 0) && Utils::IsPowerOfTwo(int_value);
+}
+
+
 RawAbstractType* BinaryMintOpInstr::CompileType() const {
   return Type::IntType();
 }