Fix type propagation rules for count operation....

test/mjsunit/compiler/loopcount.js

 // Copyright 2010 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 @@
 }
 assertEquals(0x40000000, f4());
 
 
 function f5() {
   var i;
   for (i = -0x3fffffff; i >= -0x40000000; i--) {}
   return i;
 }
 assertEquals(-0x40000001, f5());
+
+function f6() { var x = 0x3fffffff; x++; return x+1; }
+assertEquals(0x40000001, f6());