Simplify the SCoP creation and bookkeeping

  To avoid multiple exits and the resulting complicated conditions when
  creating a SCoP we now use the single hasFeasibleRuntimeContext()
  check to decide if a SCoP should be dismissed right after
  construction. If building runtime checks failed the assumed context is
  made infeasible, hence the optimized version will never be executed
  and the SCoP can be dismissed.

llvm-svn: 245593