Propagate exit conditions as described in the PET paper

Propagate exit conditions as described in the PET paper

  At some point we build loop trip counts using this method. It was replaced by
  a simpler trick that works only for affine (e.g., not modulo) constraints and
  relies on the removal of unbounded parts. In order to allow modulo constrains
  again we go back to the former, more accurate method.

llvm-svn: 247540