Fix b/122139732; update FlatAffineConstraints::isEmpty() to eliminate IDs in a

better order.

- update isEmpty() to eliminate IDs in a better order. Speed improvement for
  complex cases (for eg. high-d reshape's involving mod's/div's).
- minor efficiency update to projectOut (was earlier making an extra albeit
  benign call to gaussianEliminateIds) (NFC).
- move getBestIdToEliminate further up in the file (NFC).
- add the failing test case.
- add debug info to checkMemRefAccessDependence.

PiperOrigin-RevId: 227244634