Expression flattening improvement - reuse local expressions.

- if a local id was already for a specific mod/div expression, just reuse it if
  the expression repeats (instead of adding a new one).
- drastically reduces the number of local variables added during flattening for
  real use cases - since the same div's and mod expressions often repeat.
- add getFlattenedAffineExprs for AffineMap, IntegerSet based on the above

As a natural result of the above:

- FlatAffineConstraints(IntegerSet) ctor now deals with integer sets that have mod
  and div constraints as well, and these get simplified as well from -simplify-affine-structures

PiperOrigin-RevId: 225452174