[MLIR] FlatAffineConstraints: Refactored computation of explicit representation for identifiers

This patch refactors the existing implementation of computing an explicit
representation of an identifier as a floordiv in terms of other identifiers and
exposes this computation as a public function.

The computation of this representation is required to support local identifiers
in PresburgerSet subtract, complement and isEqual.

Reviewed By: bondhugula, arjunp

Differential Revision: https://reviews.llvm.org/D106662