Allow non-affine control flow -- SCoP Modeling

  This allows us to model non-affine regions in the SCoP representation.
  SCoP statements can now describe either basic blocks or non-affine
  regions. In the latter case all accesses in the region are accumulated
  for the statement and write accesses, except in the entry, have to be
  marked as may-write.

Differential Revision: http://reviews.llvm.org/D7846

llvm-svn: 230329