[mlir][Analysis][NFC] Split FlatAffineValueConstraints into multiple classes

The new class hierarchy is as follows:

* `IntegerRelation` (no change)
* `IntegerPolyhedron` (no change)
* `FlatLinearConstraints`: provides an AffineExpr-based API
* `FlatLinearValueConstraints`: stores an additional mapping of non-local vars to SSA values
* `FlatAffineValueConstraints`: provides additional helper functions for Affine dialect ops
* `FlatAffineRelation` (no change)

`FlatConstraints` and `FlatValueConstraints` are moved from `MLIRAffineAnalysis` to `MLIRAnalysis` and can be used without depending on the Affine dialect.

This change is in preparation of D145681, which adds an MLIR interface that depends on `FlatConstraints` (and cannot depend on the Affine dialect or any other dialect).

Differential Revision: https://reviews.llvm.org/D146201