Add early exits for SCoPs we did not optimize

  This allows us to skip ast and code generation if we did not optimize
  a SCoP and will not generate parallel or alias annotations. The
  initial heuristic to exit is simple but allows improvements later on.

  All failing test cases have been modified to disable early exit, thus
  to keep their coverage.

  Differential Revision: http://reviews.llvm.org/D7254

llvm-svn: 228851