Allow Sort to share the buffer with the operand if it is the only user.

The BitonicSort algorithm works in-place, so we can make use of that.
On GPU, so far we copied the operand to the output and then performed the algorithm in-place.
Now, we may not need to do this anymore if we see that the buffer is shared.
Also, we now only need device-to-device copies in case the buffer is not shared,
because constants are now also assigned a buffer.

PiperOrigin-RevId: 206745686