One-sided compound ref prediction is used only when all reference
frames are one-sided.

This patch has demonstrated an encoder speedup of ~28%.

Using the following configure setups, the coding performance has been
dropped on Google test sets (50 frames) in BDRate by ~0.2% for lowres
and by ~0.1% for midres (Corresponding performance impact should be
smaller on AWCY):
--enable-experimental --disable-convolve-round --disable-ext-partition
--disable-ext-partition-types --disable-txk-sel --disable-txm

Change-Id: I585bbffb2f8d154e8f52a1e79a84eff8bb4a471d