[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/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) &&