Since the CONFIG_EXT_INTER #if/#endif lines have been removed, it's a
bit clearer what's going on here and this patch cleans up the code.

Firstly, the patch pulls the cheap checks on best_mbmode.ref_frame out
to the front of the block, so we needn't call gm_get_motion_vector at
all for compound predictions.

Next, second element of the zeromv array is never used, so we needn't
compute it.

Finally, the patch removes the calls to lower_mv_precision. These
shouldn't be needed, but it's not exactly obvious why not so the patch
adds some comments to gm_get_motion_vector to explain what's going on
and adds an assertion to make sure they are true. It also adds a call
to integer_mv_precision on the early return path of
gm_get_motion_vector, correcting an apparent bug when CONFIG_AMVR is
true.

This patch shouldn't make any difference to encoder or decoder
behaviour.

Change-Id: I0b4a01063574d080bbf6d30187f4e1748c60939d