Extend/complete dependence tester to utilize local var info.

- extend/complete dependence tester to utilize local var info while adding
  access function equality constraints; one more step closer to get slicing
  based fusion working in the general case of affine_apply's involving mod's/div's.
- update test case to reflect more accurate dependence information; remove
  inaccurate comment on test case mod_deps.
- fix a minor "bug" in equality addition in addMemRefAccessConstraints (doesn't
  affect correctness, but the fixed version is more intuitive).
- some more surrounding code clean up
- move simplifyAffineExpr out of anonymous AffineExprFlattener class - the
  latter has state, and the former should reside outside.

PiperOrigin-RevId: 227175600