Add an HLO Pass to make Sort stable.

Also add a new is_stable boolean to the Sort HLO.
The pass either adds a Iota operand to the sort op or reuses such an operand
if there is one that can be used. Then the comparison computation is modified
to add a tie breaker in case the comparison determines that the relevant values
used in the comparison computation are equal.

PiperOrigin-RevId: 233911034