[MLIR][Presburger] Make IntegerRelation::mergeLocalIds not delete duplicates

This patch modifies mergeLocalIds to not delete duplicate local ids in
`this` relation. This allows the ordering of the final local ids for `this`
to be determined more easily, which is generally required when other objects
refer to these local ids.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D123866