Expose the SCEVAffinator and make it a member of a SCoP.

  This change has three major advantages:
    - The ScopInfo becomes smaller.
    - It allows to use the SCEVAffinator from outside the ScopInfo.
    - A member object allows state which in turn allows e.g., caching.

Differential Revision: http://reviews.llvm.org/D9099

llvm-svn: 244730