[MLIR][FlatAffineConstraints][NFC] Move some static functions to be available to Presburger/

This patch moves some static functions from AffineStructures.cpp to
Presburger/Utils.cpp and some to be private members of FlatAffineConstraints
(which will later be moved to IntegerPolyhedron) to allow for a smoother
transition for moving FlatAffineConstraints math functionality to
Presburger/IntegerPolyhedron.

This patch is part of a series of patches for moving math functionality to
Presburger directory.

Reviewed By: arjunp, bondhugula

Differential Revision: https://reviews.llvm.org/D115869