Add support for WritableRegister policy in the register allocator.

runtime/vm/locations.h

Index: runtime/vm/locations.h
diff --git a/runtime/vm/locations.h b/runtime/vm/locations.h
index b77deacbd637c476854633a0c657b4e5eb7f7bfb..2e8fc9c2a01e968630e7541a685a2295f3db20e0 100644
--- a/runtime/vm/locations.h
+++ b/runtime/vm/locations.h
@@ -96,12 +96,12 @@ class Location : public ValueObject {
   }
 
   // Unallocated locations.
-  // TODO(vegorov): writable register policy?
   enum Policy {
     kAny,
     kPrefersRegister,
     kRequiresRegister,
     kRequiresXmmRegister,
+    kWritableRegister,
     kSameAsFirstInput,
   };
 
@@ -134,6 +134,10 @@ class Location : public ValueObject {
     return UnallocatedLocation(kRequiresXmmRegister);
   }
 
+  static Location WritableRegister() {
+    return UnallocatedLocation(kWritableRegister);
+  }
+
   // The location of the first input to the instruction will be
   // used to replace this unallocated location.
   static Location SameAsFirstInput() {