[XLA:TF] Enable int8 and uint8 support in the bridge for CPU/GPU

The test changes are awkward. None of these are XLA bugs, it's just that the op
definitions in tensorflow are really inconsistent. I tried to infer whether the
limitation is on signed types, index types or just arbitrary. In the latter
case just int8/uint8 is blacklisted, we should probably lift that requirement
at some point.

PiperOrigin-RevId: 213243906