Skip redundant compound search

Skip compound mode search if reference_mode is SINGLE_REFERENCE.

Tested foreman, city, student CIF (30 frames, bitrate=500),
CPU3 speed up about 1.5%
CPU7&8 speed up about 30%

Change-Id: I270edf5f952eda670e2dd557a5874786ba8764d4

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index f829cc6..ded3a0b 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -10024,6 +10024,8 @@
   if (comp_pred) {
     if (!cpi->allow_comp_inter_inter) return 1;
 
+    if (cm->reference_mode == SINGLE_REFERENCE) return 1;
+
     // Skip compound inter modes if ARF is not available.
     if (!(cpi->ref_frame_flags & ref_frame_flag_list[ref_frame[1]])) return 1;