Fix affine expr flattener bug + improve simplification in a particular scenario

- fix visitDivExpr: constraints constructed for localVarCst used the original
  divisor instead of the simplified divisor; fix this. Add a simple test case
  in memref-bound-check that reproduces this bug - although this was encountered in the
  context of slicing for fusion.

- improve mod expr flattening: when flattening mod expressions,
  cancel out the GCD of the numerator and denominator so that we can get a
  simpler flattened form along with a simpler floordiv local var for it

PiperOrigin-RevId: 228539928