[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

Author: webmiche

mlir/include/mlir/Analysis/Presburger/PresburgerRelation.h

@@ -18,6 +18,11 @@
 namespace mlir {
 namespace presburger {
 
+/// The SetCoalescer class contains all functionality concerning the coalesce
+/// heuristic. It is built from a `PresburgerRelation` and has the `coalesce()`
+/// function as its main API.
+class SetCoalescer;
+
 /// A PresburgerRelation represents a union of IntegerRelations that live in
 /// the same PresburgerSpace with support for union, intersection, subtraction,
 /// and complement operations, as well as sampling.
@@ -120,6 +125,8 @@ class PresburgerRelation : public PresburgerSpace {
 
   /// The list of disjuncts that this set is the union of.
   SmallVector<IntegerRelation, 2> integerRelations;
+
+  friend class SetCoalescer;
 };
 
 class PresburgerSet : public PresburgerRelation {