Also eliminate map checks with transitions.

test/mjsunit/omit-constant-mapcheck.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 35 matching lines @@
   return g2.a;
 }
 
 assertEquals(2, load2());
 assertEquals(2, load2());
 %OptimizeFunctionOnNextCall(load2);
 assertEquals(2, load2());
 g2.b = 10;
 g2.a = 5;
 assertEquals(5, load2());
+
+var g3 = { a:2, b:9, c:1 }
+
+function store(v) {
+  g3.a = v;
+  return g3.a;
+}
+
+assertEquals(5, store(5));
+assertEquals(8, store(8));
+%OptimizeFunctionOnNextCall(store);
+assertEquals(10, store(10));
+delete g3.c;
+store(7);
+assertEquals({a:7, b:9}, g3);