Make comp_inter_inter support 4x4 partition coding
Make comp_inter_inter support 4x4 partition coding

This commit refactors the iterative motion search for compound
inter-inter mode, to make it support all partition types including
4x4/4x8/8x4 block sizes.

Change-Id: I5f1212b0f307377291763e45c6bdc9693b5f04c8

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