[MLIR][Presburger] Attach values only to non-local identifiers in FAVC

This patch changes `FlatAffineValueConstraints` to only allow attaching
values to non-local identifiers.

The reasoning for this change is:
1. Information attached to local identifiers can be lost since local identifiers
  can be removed for output size optimizations.
2. There are no current use cases for attaching values to Local identifiers.
3. Attaching a value to a local identifier does not make sense since a local
  identifier represents existential quantification.

This patch also adds some additional asserts to the affected functions.

Reviewed By: arjunp, bondhugula

Differential Revision: https://reviews.llvm.org/D125613