Fixed bug in while-loops that caused an assertion to fail.

Fixed bug in while-loops that caused an assertion to fail.

We forgot resetting true-/false-label to NULL after evaluating the
condition expression in dowhile- and while-loops.
This change fixes this.

This causes an assertion to fail in VisitIfStatement whenever there is an
if-statement after a while-loop before. e.g. like in:

  var i=0, j=0;
  while(j<5) { j++; }
  if (i ==0 ) { j++; }

Review URL: http://codereview.chromium.org/371070

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3255 ce2b1a6d-e550-0410-aec6-3dcde31c8c00