Decouple SCoP building logic from pass

  Created a new pass ScopInfoRegionPass. As name suggests, it is a
  region pass and it is there to preserve compatibility with our
  existing Polly passes.  ScopInfoRegionPass will return a SCoP object
  for a valid region while the creation of the SCoP stays in the
  ScopInfo class.

Contributed-by: Utpal Bora <cs14mtech11017@iith.ac.in>
Reviewed-by: Tobias Grosser <tobias@grosser.es&gt;,>
               Johannes Doerfert <doerfert@cs.uni-saarland.de>

Differential Revision: http://reviews.llvm.org/D20770

llvm-svn: 271259