[MLIR] Add flat affine constraints method to round trip integer set

- add method to get back an integer set from flat affine constraints;
  this allows a round trip
- use this to complete the simplification of integer sets in
  -simplify-affine-structures
- update FlatAffineConstraints::removeTrivialRedundancy to also do GCD
  tightening and normalize by GCD (while still keeping it linear time).

Signed-off-by: Uday Bondhugula <uday@polymagelabs.com>