[MLIR][Affine] Expose getAffineScope as a utility function

The getAffineScope function is currently internal
to AffineOps.cpp. However, as the comment on the function
itself notes, this is useful in a variety of other places
externally. This PR allows other files to use the function.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D121827