[MLIR] Introduce coalesce for PresburgerSet

This patch provides functionality for simplifying `PresburgerSet`s by checking if any `FlatAffineConstraints` in the set is contained in another, and removing such redundant FACs.

This is part of a series of patches to provide functionality for [integer set coalescing](http://impact.gforge.inria.fr/impact2015/papers/impact2015-verdoolaege.pdf) in MLIR.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D110617