Eliminating usage of allow_comp_inter_inter in the decoder.

Splitting setup_inter_inter function into is_compound_prediction_allowed
and setup_compound_prediction. Moving setup_compound_prediction call
into read_comp_pred from read_uncompressed_header.

We should do the same in the encoder as well.

Change-Id: I40d75fdc4a221b2f7705df00d23a4b3fe79987c3