[debugger] fix break locations for assignments and return.

test/mjsunit/debug-step.js

 // Copyright 2008 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 46 matching lines @@
 function f() {
   var i;                        // Line 1.
   for (i = 0; i < 1000; i++) {  // Line 2.
     x = 1;                      // Line 3.
   }
 };
 
 // Set a breakpoint on the for statement (line 1).
 bp1 = Debug.setBreakPoint(f, 1);
 
-// Check that performing 1000 steps will make i 499.
+// Check that performing 1000 steps will make i 333.
 var step_count = 1000;
 result = -1;
 f();
-assertEquals(332, result);
+assertEquals(333, result);
 Debug.setListener(null);