[MLIR] Add mergeLocalIds and mergeSymbolIds

This patch adds mergeLocalIds andmergeSymbolIds as public functions
for FlatAffineConstraints and FlatAffineValueConstraints respectively.

mergeLocalIds is also required to support divisions in intersection,
subtraction, equality checks, and complement for PresburgerSet.

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

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D110045