[MLIR] FlatAffineConstraints: Use Matrix objects to store the constraints

This results in significant deduplication of code. This patch is not expected to change any functionality, it's just some simplification in preparation for future work. Also slightly simplified some code that was being touched anyway and added some unit tests for some functions that were touched.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D105152