Canonicalize affine set + operands while adding affine.if op domain

Canonicalize affine set + operands in addAffineIfOpDomain. This is to
ensure a unique set of operands for FlatAffineValueConstraints and in
general to provide a simplified set of constraints. For the latter
scenario, this just leads to efficiency improvements as opposed to
functionality. While on this, remove outdated/stale stuff from
AffineStructures.h.

Fixes: https://github.com/llvm/llvm-project/issues/59461

Differential Revision: https://reviews.llvm.org/D141250