Update addSliceBounds to deal with loops with floor's/mod's.

- This change only impacts the cost model for fusion, given the way
  addSliceBounds was being used. It so happens that the output in spite of this
  CL's fix is the same; however, the assertions added no longer fail. (an
  invalid/inconsistent memref region was being used earlier).

PiperOrigin-RevId: 236405030