[mlir][affine][analysis][NFC] Simplify FlatAffineConstraints API

* Remove `reset` function. Use copy assignment directly (instead of within `reset`).
* Fix potential `nullptr` dereference in `getFlattenedAffineExprs`.
* Make constraint set optional in `checkMemrefAccessDependence`.

Differential Revision: https://reviews.llvm.org/D145935