[MLIR] Add functionality to remove redundant local variables

This patch adds functionality to FlatAffineConstraints to remove local
variables using equalities. This helps in keeping output representation of
FlatAffineConstraints smaller.

This patch is part of a series of patches aimed at generalizing affine
dependence analysis.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D110056