[MLIR][Presburger] Support divisions in union of two PWMAFunction

This patch adds support for divisions in the union of two PWMAFunction. This is
now possible because of previous patches, which made divisions explicitly
stored in MultiAffineFunction (MAF). This patch also refactors the previous
implementation, moving the implementation for obtaining a set of points where a
MAF is lexicographically "better" than the other to MAF.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D138118