This patch modifies the interpretation of SEG_LVL_SKIP and
SEG_LVL_ZEROMV slightly, to fix a decoder crash and to save bits
in the intended use cases of these segment flags.

Previously, blocks using either of these segment flags could
signal reference frames just like any other block. But the mode
was implicitly taken to be ZEROMV. This worked fine in VP9, but
crashed for compound blocks in AV1 since those should use
ZERO_ZEROMV instead.

Now we make it so that SEG_LVL_SKIP and SEG_LVL_ZEROMV imply
that the block is single-reference. The reference to use is taken
from the SEG_LVL_REF_FRAME segment feature if that is present,
or is set to LAST_FRAME if not. See the attached bug report
for the reasoning behind this.

As a related change, we also teach the encoder how to deal with
the combination of SEG_LVL_SKIP + SEG_LVL_REF_FRAME.

BUG=aomedia:675

Change-Id: I5e657cbfc1f08395a0301cba701edfb1682502a5