[MLIR][Presburger] introduce SetCoalescer

This patch refactors the current coalesce implementation. It introduces
the `SetCoalescer`, a class in which all coalescing functionality lives.
The main advantage over the old design is the fact that the vectors of
constraints do not have to be passed around, but are implemented as
private fields of the SetCoalescer. This will become especially
important once more inequality types are introduced.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D121364