Upgrade/fix/simplify store to load forwarding

- fix store to load forwarding for a certain set of cases (where
  forwarding shouldn't have happened); use AffineValueMap difference
  based MemRefAccess equality checking; utility logic is also greatly
  simplified

- add missing equality/inequality operators for AffineExpr ==/!= ints

- add == != operators on MemRefAccess

Closes tensorflow/mlir#136

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/136 from bondhugula:store-load-forwarding d79fd1add8bcfbd9fa71d841a6a9905340dcd792
PiperOrigin-RevId: 270457011