Temp change in FlatAffineConstraints::getSliceBounds() to deal with TODO in

LoopFusion

- getConstDifference in LoopFusion is pending a refactoring to handle bounds
  with min's and max's; it currently asserts on some useful test cases that we
  want to experiment with. This CL changes getSliceBounds to be more
  conservative so as to not trigger the assertion. Filed b/126426796 to track this.

PiperOrigin-RevId: 235826538