dart2js cps: Store the TypeMask for each primitive in a field.

pkg/compiler/lib/src/cps_ir/loop_invariant_code_motion.dart

 // Copyright (c) 2015, 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.
 
 library dart2js.cps_ir.loop_invariant_code_motion;
 
 import 'optimizers.dart';
 import 'cps_ir_nodes.dart';
 import 'loop_hierarchy.dart';
 
@@ skipping 108 matching lines @@
     if (loop1 == null) return loop2;
     if (loop2 == null) return loop1;
     if (loopHierarchy.loopDepth[loop1] > loopHierarchy.loopDepth[loop2]) {
       return loop1;
     } else {
       return loop2;
     }
   }
 
   bool shouldLift(Primitive prim) {
-    // Interceptors are safe and almost always profitable for lifting
+    // Interceptors are generally safe and almost always profitable for lifting
     // out of loops. Several other primitive could be lifted too, but it's not
     // always profitable to do so.
+
+    // Note that the type of the interceptor is technically not sound after
+    // lifting because its current type might have been computed based on a
+    // refinement node (which has since been removed). As a whole, it still
+    // works out because all uses of the interceptor must occur in a context
+    // where it indeed has the type it claims to have.
     return prim is Interceptor;
   }
 }