Add support for WritableRegister policy in the register allocator.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 349fae043e8748a9ac110f9bca2ac7feafff515e..5c69770bb10031c6df6d2d0b636b161b05a44ee5 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -1534,20 +1534,24 @@ void ChainContextInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
 
 
 LocationSummary* StoreVMFieldInstr::MakeLocationSummary() const {
-  return LocationSummary::Make(
-      2,
-      is_used() ? Location::SameAsFirstInput() : Location::NoLocation(),
-      LocationSummary::kNoCall);
+  const intptr_t kNumInputs = 2;
+  const intptr_t kNumTemps = 0;
+  LocationSummary* locs =
+      new LocationSummary(kNumInputs, kNumTemps, LocationSummary::kNoCall);
+  locs->set_in(0, value()->NeedsStoreBuffer() ? Location::WritableRegister()
+                                              : Location::RequiresRegister());
+  locs->set_in(1, Location::RequiresRegister());
+  return locs;
 }
 
 
 void StoreVMFieldInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   Register value_reg = locs()->in(0).reg();
   Register dest_reg = locs()->in(1).reg();
-  ASSERT(!is_used() || (value_reg == locs()->out().reg()));
 
   if (value()->NeedsStoreBuffer()) {
-    __ StoreIntoObject(dest_reg, FieldAddress(dest_reg, offset_in_bytes()),
+    __ StoreIntoObject(dest_reg,
+                       FieldAddress(dest_reg, offset_in_bytes()),
                        value_reg);
   } else {
     __ StoreIntoObjectNoBarrier(