[MLIR][NFC] FlatAffineConstraints: Refactor division representation computation

This patch factors out division representation computation from upper-lower bound
inequalities to a separate function. This is done to improve readability and reuse.

This patch is marked NFC since the only change is factoring out existing code
to a separate function.

Reviewed By: grosser

Differential Revision: https://reviews.llvm.org/D113463