[MLIR] FlatAffineConstraints: Allow extraction of explicit representation of local variables

This patch extends the existing functionality of computing an explicit
representation for local variables, to also get the explicit representation,
instead of only the inequality pairs.

This is required for a future patch to remove redundant local ids based on
their explicit representation.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D113814