Add support for WritableRegister policy in the register allocator.

runtime/vm/locations.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/locations.h"
 
 #include "vm/il_printer.h"
 #include "vm/intermediate_language.h"
 #include "vm/flow_graph_compiler.h"
 
@@ skipping 60 matching lines @@
     case kUnallocated:
       switch (policy()) {
         case kAny:
           return "A";
         case kPrefersRegister:
           return "P";
         case kRequiresRegister:
           return "R";
         case kRequiresXmmRegister:
           return "DR";
+        case kWritableRegister:
+          return "WR";
         case kSameAsFirstInput:
           return "0";
       }
       UNREACHABLE();
     default:
       ASSERT(IsConstant());
       return "C";
   }
   return "?";
 }
@@ skipping 40 matching lines @@
 
   if (!out().IsInvalid()) {
     f->Print(" => ");
     out().PrintTo(f);
   }
 
   if (always_calls()) f->Print(" C");
 }
 
 }  // namespace dart