Fix mismatch in global-motion experiment

This was introduced in 72e2e982ee

Change-Id: Ia4d0a0606a518964d5f6f257874f8f3d5b4d651b

av1/decoder/decodemv.c

@@ -1695,15 +1695,16 @@ static void read_inter_block_mode_info(AV1Decoder *const pbi,
 #else
       zeromv[0].as_int = zeromv[1].as_int = 0;
 #endif
-      for (ref = 0; ref < 2; ++ref) {
-        if (rf[ref] == NONE_FRAME) continue;
+      for (ref = 0; ref < 1 + is_compound; ++ref) {
         lower_mv_precision(&ref_mvs[rf[ref]][0].as_mv, allow_hp);
         lower_mv_precision(&ref_mvs[rf[ref]][1].as_mv, allow_hp);
-
-        if (ref_mvs[rf[ref]][0].as_int != zeromv[0].as_int ||
-            ref_mvs[rf[ref]][1].as_int != zeromv[1].as_int)
-          inter_mode_ctx[ref_frame] &= ~(1 << ALL_ZERO_FLAG_OFFSET);
       }
+
+      if (ref_mvs[rf[0]][0].as_int != zeromv[0].as_int ||
+          ref_mvs[rf[0]][1].as_int != zeromv[0].as_int ||
+          (is_compound && (ref_mvs[rf[1]][0].as_int != zeromv[1].as_int ||
+                           ref_mvs[rf[1]][1].as_int != zeromv[1].as_int)))
+        inter_mode_ctx[ref_frame] &= ~(1 << ALL_ZERO_FLAG_OFFSET);
     }
   }