[MLIR][Presburger] remove redundant constraints in coalesce

This patch improves the representation size of individual
`IntegerRelation`s by calling the function
`IntegerRelation::removeRedundantConstraints`. While this is only a
slight optimization in the current version, it will be necessary for
patches to come.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D121989