[MLIR] FlatAffineConstraints: Ensure dimensionalities match when calling mergeLocalIds

This patch reorders mergeLocalIds usage to merge locals only after number of
dimensions and symbols are same. This does not change any functionality
because it does not matter in what order identifiers are merged, since
the reason to do it is to ensure that two FACs are aligned.

The order ensured in this patch simplifies a subsequent patch to improve
mergeLocalIds which requires dimensions and symbols to be aligned.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D112841