Add new compound modes using single reference

The hookup with the encoder-decoder pipeline will be in the following
CLs. This is under the experiment of compound-singleref, with ext-
inter enabled.

Change-Id: I6523a1a6525539eb7c6aa6c5e11724a694e8bf6c
diff --git a/av1/common/pred_common.h b/av1/common/pred_common.h
index 7825f3c..e16ad70 100644
--- a/av1/common/pred_common.h
+++ b/av1/common/pred_common.h
@@ -168,6 +168,15 @@
 }
 #endif  // CONFIG_EXT_REFS
 
+#if CONFIG_EXT_INTER && CONFIG_COMPOUND_SINGLEREF
+int av1_get_inter_mode_context(const MACROBLOCKD *xd);
+
+static INLINE aom_prob av1_get_inter_mode_prob(const AV1_COMMON *cm,
+                                               const MACROBLOCKD *xd) {
+  return cm->fc->comp_inter_mode_prob[av1_get_inter_mode_context(xd)];
+}
+#endif  // CONFIG_EXT_INTER && CONFIG_COMPOUND_SINGLEREF
+
 // Returns a context number for the given MB prediction signal
 // The mode info data structure has a one element border above and to the
 // left of the entries corresponding to real blocks.