Complete AffineExprFlattener based simplification for floordiv/ceildiv.

- handle floordiv/ceildiv in AffineExprFlattener; update the simplification to
  work even if mod/floordiv/ceildiv expressions appearing in the tree can't be eliminated.
- refactor the flattening / analysis to move it out of lib/Transforms/
- fix MutableAffineMap::isMultipleOf
- add AffineBinaryOpExpr:getAdd/getMul/... utility methods

PiperOrigin-RevId: 211540536