Add a new experiment of REF_ADAPT

Noticed that some ALTREF_FRAMEs could have used compound modes for its
prediction but have been labeled as SINGLE_REFERENCE mode in the frame
header. This experiment is to remove the COMPOUND_REFERENCE mode from
the frame-level reference mode choices and only leave SINGLE_REFERENCE
and REFERENCE_MODE_SELECT the two choices in the frame header.

When turning on both ext-refs and ref-adapt, compared against ext-refs
itself, a small gain is achieved. In PSNR, the bitrate saving gains are
as follows:

lowres: Avg -0.120%; BDRate -0.128%
midres: Avg -0.155%; BDRate -0.128%

Change-Id: I2cfff8a6b7eaa65ef863dbdbc4dd086d3b586f8c

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 4b9f4dc..b363417 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -4462,10 +4462,14 @@
 
   if (cpi->allow_comp_inter_inter) {
     const int use_hybrid_pred = cm->reference_mode == REFERENCE_MODE_SELECT;
+#if !CONFIG_REF_ADAPT
     const int use_compound_pred = cm->reference_mode != SINGLE_REFERENCE;
+#endif  // !CONFIG_REF_ADAPT
 
     aom_wb_write_bit(wb, use_hybrid_pred);
+#if !CONFIG_REF_ADAPT
     if (!use_hybrid_pred) aom_wb_write_bit(wb, use_compound_pred);
+#endif  // !CONFIG_REF_ADAPT
   }
 
   write_tile_info(cm, wb);