[XLA:GPU] In Multi-output fusion, use BFS to check reachability
[XLA:GPU] In Multi-output fusion, use BFS to check reachability

Currently, MultiOutputFusion uses HloReachabilityMap, which builds reachability matrix on creation and element lookup on request. With multi-output fusion rebuilding the reachability after every step, that may take time.

This change makes MultiOutputFusion to use a simple breadth-first search reachability check, bounded by the post-order index. Namely,  
* To build the "map", the instructions are sorted in the post order, and indices are stored.  
* On the request, the BFS from the destination node is started, and only goes through nodes which have post-order index >= source node.

With this approach, the MultiOutputFusion pass of one example HLO module goes from 34 minutes to 4 minutes. There are more advanced data structures that help even more (0.5 minutes), this will be done in the subsequent changes.
