Do not model scalar accesses in non-affine subregions

  If a scalar was defined and used only in a non-affine subregion we do
  not need to model the accesses. However, if the scalar was defined
  inside the region and escapes the region we have to model the access.
  The same is true if the scalar was defined outside and used inside the
  region.

llvm-svn: 230960