[MLIR] Use IntegerPolyhedron in Simplex instead of FlatAffineConstraints

This patch replaces usage of FlatAffineConstraints in Simplex with
IntegerPolyhedron. This removes dependency of Simplex on FlatAffineConstraints
and puts it on IntegerPolyhedron, which is part of Presburger library.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D116287