[NORMATIVE, spatial-segmentation] Undo unintended change

Patch cff20b9 accientally made a normative change to the
spatial-segmentation experiment while applying some refactoring.
Undo the normative part of the change, while keeping the overall
refactoring.

BUG=aomedia:1625

Change-Id: I01278da2a35b9b9fdca8bb9a3fe56910a6471eda

av1/common/pred_common.h

diff --git a/av1/common/pred_common.h b/av1/common/pred_common.h
index c06571b..905b485 100644
--- a/av1/common/pred_common.h
+++ b/av1/common/pred_common.h
@@ -64,11 +64,14 @@
   }
 
   // Pick CDF index based on number of matching/out-of-bounds segment IDs.
-  *cdf_index = 0;
-  if ((prev_ul == prev_u) && (prev_ul == prev_l))
+  if (prev_ul < 0 || prev_u < 0 || prev_l < 0) /* Edge case */
+    *cdf_index = 0;
+  else if ((prev_ul == prev_u) && (prev_ul == prev_l))
     *cdf_index = 2;
   else if ((prev_ul == prev_u) || (prev_ul == prev_l) || (prev_u == prev_l))
     *cdf_index = 1;
+  else
+    *cdf_index = 0;
 
   // If 2 or more are identical returns that as predictor, otherwise prev_l.
   if (prev_u == -1)  // edge case