Refactor av1_build_inter_predictors_sb().

Before this, this function was copied-and-edited from
av1_build_inter_predictors_sby() and av1_build_inter_predictors_sbuv().
Also, some places used sb() one directly (like rdopt.c), while others used
sby() and sbuv() separately (like encodeframe.c --> encode_superblock()).

This was prone to errors, if someone mistakenly modified only one/two of
the 3 functions.

Now, we just reuse sby() and sbuv() functions in sb().

Verified that there's no change in output.

Change-Id: If81d78e767cc3fa09f06094d4db36ff1eaea3e4f

av1/common/reconinter.c

diff --git a/av1/common/reconinter.c b/av1/common/reconinter.c
index dd37850..69e469b 100644
--- a/av1/common/reconinter.c
+++ b/av1/common/reconinter.c
@@ -1400,28 +1400,11 @@
 #endif  // CONFIG_EXT_INTER && CONFIG_INTERINTRA
 }
 
-// TODO(afergs): Check if ctx can be made constant
 void av1_build_inter_predictors_sb(const AV1_COMMON *cm, MACROBLOCKD *xd,
                                    int mi_row, int mi_col, BUFFER_SET *ctx,
                                    BLOCK_SIZE bsize) {
-  build_inter_predictors_for_planes(cm, xd, bsize, mi_row, mi_col, 0,
-                                    MAX_MB_PLANE - 1);
-#if CONFIG_EXT_INTER && CONFIG_INTERINTRA
-  if (is_interintra_pred(&xd->mi[0]->mbmi)) {
-    BUFFER_SET default_ctx = {
-      { xd->plane[0].dst.buf, xd->plane[1].dst.buf, xd->plane[2].dst.buf },
-      { xd->plane[0].dst.stride, xd->plane[1].dst.stride,
-        xd->plane[2].dst.stride }
-    };
-    if (!ctx) ctx = &default_ctx;
-    av1_build_interintra_predictors(
-        xd, xd->plane[0].dst.buf, xd->plane[1].dst.buf, xd->plane[2].dst.buf,
-        xd->plane[0].dst.stride, xd->plane[1].dst.stride,
-        xd->plane[2].dst.stride, ctx, bsize);
-  }
-#else
-  (void)ctx;
-#endif  // CONFIG_EXT_INTER && CONFIG_INTERINTRA
+  av1_build_inter_predictors_sby(cm, xd, mi_row, mi_col, ctx, bsize);
+  av1_build_inter_predictors_sbuv(cm, xd, mi_row, mi_col, ctx, bsize);
 }
 
 void av1_setup_dst_planes(struct macroblockd_plane planes[MAX_MB_PLANE],

av1/encoder/encodeframe.c

diff --git a/av1/encoder/encodeframe.c b/av1/encoder/encodeframe.c
index 8a65d88..5e7975a 100644
--- a/av1/encoder/encodeframe.c
+++ b/av1/encoder/encodeframe.c
@@ -6020,8 +6020,7 @@
     }
 #endif  // CONFIG_EXT_INTER && CONFIG_COMPOUND_SINGLEREF
 
-    av1_build_inter_predictors_sby(cm, xd, mi_row, mi_col, NULL, block_size);
-    av1_build_inter_predictors_sbuv(cm, xd, mi_row, mi_col, NULL, block_size);
+    av1_build_inter_predictors_sb(cm, xd, mi_row, mi_col, NULL, block_size);
 #if CONFIG_MOTION_VAR
     if (mbmi->motion_mode == OBMC_CAUSAL) {
 #if CONFIG_NCOBMC