[FIX] Approximate non-affine loops correctly

  Before isValidCFG() could hide the fact that a loop is non-affine by
  over-approximation. This is problematic if a subregion of the loop contains
  an exit/latch block and is over-approximated. Now we do not over-approximate
  in the isValidCFG function if we check loop control.  If such control is
  non-affine the whole loop is over-approximated, not only a subregion.

llvm-svn: 249273