Improve register usage for StoreInstanceField.

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
===================================================================
--- runtime/vm/intermediate_language_x64.cc	(revision 12307)
+++ runtime/vm/intermediate_language_x64.cc	(working copy)
@@ -972,22 +972,28 @@
   LocationSummary* summary =
       new LocationSummary(kNumInputs, num_temps, LocationSummary::kNoCall);
   summary->set_in(0, Location::RequiresRegister());
-  summary->set_in(1,
-      value()->NeedsStoreBuffer() ? Location::WritableRegister()
-                                  : Location::RequiresRegister());
+  summary->set_in(1, (value()->NeedsStoreBuffer() && emit_store_barrier())
+                       ? Location::WritableRegister()
+                       : Location::RegisterOrConstant(value()));
   return summary;
 }
 
 
 void StoreInstanceFieldInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   Register instance_reg = locs()->in(0).reg();
-  Register value_reg = locs()->in(1).reg();
   if (this->value()->NeedsStoreBuffer() && emit_store_barrier()) {
+    Register value_reg = locs()->in(1).reg();
     __ StoreIntoObject(instance_reg,
         FieldAddress(instance_reg, field().Offset()), value_reg);
   } else {
-    __ StoreIntoObjectNoBarrier(instance_reg,
-        FieldAddress(instance_reg, field().Offset()), value_reg);
+    if (locs()->in(1).IsConstant()) {
+      __ StoreObject(FieldAddress(instance_reg, field().Offset()),
+                     locs()->in(1).constant());
+    } else {
+      Register value_reg = locs()->in(1).reg();
+      __ StoreIntoObjectNoBarrier(instance_reg,
+          FieldAddress(instance_reg, field().Offset()), value_reg);
+    }
   }
 }