Changed code to remove extra read/write loops when not necessary

Modified code so that:
-When above and left contexts are same and not equal to current segment id, it needs to read a maximum of 2 segment_tree_probabilities.
- When above and left contexts are different and not equal to current segment id, it needs to read only a single segment_tree_probability.

Change-Id: Idc2cf2c4afcc6179b8162ac5a32c948ff5a9a2ba