Avoid sending bits for the compound type for sub 8x8 blocks

The only compound mode used with sub 8x8 blocks is COMPOUND_AVERAGE, so
we don't have to send anything in this case

Change-Id: I90d0162e5f7f1ad205e65094293cde2a48eb77b1
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 83a2278..4b9f4dc 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -1714,7 +1714,7 @@
 #if CONFIG_MOTION_VAR
         && mbmi->motion_mode == SIMPLE_TRANSLATION
 #endif  // CONFIG_MOTION_VAR
-        ) {
+        && is_any_masked_compound_used(bsize)) {
       av1_write_token(
           w, av1_compound_type_tree, cm->fc->compound_type_prob[bsize],
           &compound_type_encodings[mbmi->interinter_compound_data.type]);