[NORMATIVE, jnt-comp] Disable compound_idx if !enable_jnt_comp

If enable_jnt_comp is 0, the code still signals the compound_idx
symbol, but both choices result in the exact same compound prediction.
This patch fixes that redundancy by forcing compound_idx = 1
whenever enable_jnt_comp = 0.

Also add an assert in bitstream.c, to make sure that the encoder
always selects the correct value of compound_idx.

BUG=aomedia:1442

Change-Id: I905052f7f85c53eadbd6ba43c905dabd4c3cb65c

av1/common/reconinter.c

diff --git a/av1/common/reconinter.c b/av1/common/reconinter.c
index 0b08503..fa47f5f 100644
--- a/av1/common/reconinter.c
+++ b/av1/common/reconinter.c
@@ -870,7 +870,7 @@
                                 int order_idx, int *fwd_offset, int *bck_offset,
                                 int *use_jnt_comp_avg, int is_compound) {
   assert(fwd_offset != NULL && bck_offset != NULL);
-  if (!is_compound || mbmi->compound_idx || !cm->seq_params.enable_jnt_comp) {
+  if (!is_compound || mbmi->compound_idx) {
     *use_jnt_comp_avg = 0;
     return;
   }

av1/decoder/decodemv.c

diff --git a/av1/decoder/decodemv.c b/av1/decoder/decodemv.c
index 40833bd..d6bfc4d 100644
--- a/av1/decoder/decodemv.c
+++ b/av1/decoder/decodemv.c
@@ -1930,9 +1930,14 @@
     }
 
     if (mbmi->comp_group_idx == 0) {
-      const int comp_index_ctx = get_comp_index_context(cm, xd);
-      mbmi->compound_idx = aom_read_symbol(
-          r, ec_ctx->compound_index_cdf[comp_index_ctx], 2, ACCT_STR);
+      if (cm->seq_params.enable_jnt_comp) {
+        const int comp_index_ctx = get_comp_index_context(cm, xd);
+        mbmi->compound_idx = aom_read_symbol(
+            r, ec_ctx->compound_index_cdf[comp_index_ctx], 2, ACCT_STR);
+      } else {
+        // Distance-weighted compound is disabled, so always use average
+        mbmi->compound_idx = 1;
+      }
     } else {
       assert(cm->reference_mode != SINGLE_REFERENCE &&
              is_inter_compound_mode(mbmi->mode) &&

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 6220340..c461273 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -1290,9 +1290,13 @@
         if (mbmi->compound_idx)
           assert(mbmi->interinter_compound_type == COMPOUND_AVERAGE);
 
-        const int comp_index_ctx = get_comp_index_context(cm, xd);
-        aom_write_symbol(w, mbmi->compound_idx,
-                         ec_ctx->compound_index_cdf[comp_index_ctx], 2);
+        if (cm->seq_params.enable_jnt_comp) {
+          const int comp_index_ctx = get_comp_index_context(cm, xd);
+          aom_write_symbol(w, mbmi->compound_idx,
+                           ec_ctx->compound_index_cdf[comp_index_ctx], 2);
+        } else {
+          assert(mbmi->compound_idx == 1);
+        }
       } else {
         assert(cpi->common.reference_mode != SINGLE_REFERENCE &&
                is_inter_compound_mode(mbmi->mode) &&

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 3c9bd7a..ba4bd63 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -8136,10 +8136,11 @@
   const MB_MODE_INFO backup_mbmi = *mbmi;
   MB_MODE_INFO best_mbmi = *mbmi;
   int64_t early_terminate = 0;
-  const int search_jnt_comp = is_comp_pred & cm->seq_params.enable_jnt_comp;
 
   int comp_idx;
-  for (comp_idx = 0; comp_idx < 1 + search_jnt_comp; ++comp_idx) {
+  const int search_jnt_comp = is_comp_pred & cm->seq_params.enable_jnt_comp;
+  // If !search_jnt_comp, we need to force mbmi->compound_idx = 1.
+  for (comp_idx = !search_jnt_comp; comp_idx < 2; ++comp_idx) {
     compmode_interinter_cost = 0;
     early_terminate = 0;
     *rd_stats = backup_rd_stats;