Invalidate unprofitable SCoPs after creation

  If a profitable run is performed we will check if the SCoP seems to be
  profitable after creation but before e.g., dependence are computed. This is
  needed as SCoP detection only approximates the actual SCoP representation.
  In the end this should allow us to be less conservative during the SCoP
  detection while keeping the compile time in check.

llvm-svn: 269074