Specialize mint shift code on arm for a constant shift amount.

runtime/vm/locations.cc

Index: runtime/vm/locations.cc
===================================================================
--- runtime/vm/locations.cc	(revision 38346)
+++ runtime/vm/locations.cc	(working copy)
@@ -84,6 +84,14 @@
 }
 
 
+Location Location::WritableRegisterOrSmiConstant(Value* value) {
+  ConstantInstr* constant = value->definition()->AsConstant();
+  return ((constant != NULL) && Assembler::IsSafeSmi(constant->value()))
+      ? Location::Constant(constant->value())
+      : Location::WritableRegister();
+}
+
+
 Location Location::FixedRegisterOrConstant(Value* value, Register reg) {
   ConstantInstr* constant = value->definition()->AsConstant();
   return ((constant != NULL) && Assembler::IsSafe(constant->value()))