[XLA:TPU] Remove redundant broadcast creation in host offloader.
[XLA:TPU] Remove redundant broadcast creation in host offloader.

Currently, host_offloader duplicates all broadcasts for each user, i.e. a broadcast with 3 users becomes three separate broadcasts. This is to avoid offloaded DUS broadcasts from removing broadcasts with non-offloaded users. However, it creates a lot of redundant broadcast duplications. 

This change only duplicates broadcasts if they are fed into DUS which are offloaded.
