The refactoring serves two purposes:
1. Separate code paths for jnt_comp and original compound average
computation. It provides function interface for jnt_comp while leaving
original compound average computation unchanged. In near future, SIMD
functions can be added for jnt_comp using the interface.

2. Previous implementation uses a hack on second_pred. But it may cause
segmentation fault when the test clip is small. As reported in Issue
944. This refactoring removes hacking and make it possible to address
the seg fault problem in the future.

Change-Id: Idd2cb99f6c77dae03d32ccfa1f9cbed1d7eed067