Simplify a condition in av1_get_spatial_seg_pred.

Given that get_segment_id() returns a nonnegative value, one can show
that prev_u < 0 || prev_l < 0 is equivalent to prev_ul < 0. Therefore
we can simplify the condition prev_ul < 0 || prev_u < 0 || prev_l < 0
to prev_ul < 0.

BUG=aomedia:2327

Change-Id: Ica8f3a001bf104c010c2adf42d2945c83e0e01e3

av1/common/pred_common.h

diff --git a/av1/common/pred_common.h b/av1/common/pred_common.h
index 1d69ef0..d9b30a9 100644
--- a/av1/common/pred_common.h
+++ b/av1/common/pred_common.h
@@ -59,9 +59,13 @@
     prev_l = get_segment_id(cm, cm->cur_frame->seg_map, BLOCK_4X4, mi_row - 0,
                             mi_col - 1);
   }
+  // This property follows from the fact that get_segment_id() returns a
+  // nonnegative value. This allows us to test for all edge cases with a simple
+  // prev_ul < 0 check.
+  assert(IMPLIES(prev_ul >= 0, prev_u >= 0 && prev_l >= 0));
 
   // Pick CDF index based on number of matching/out-of-bounds segment IDs.
-  if (prev_ul < 0 || prev_u < 0 || prev_l < 0) /* Edge case */
+  if (prev_ul < 0) /* Edge cases */
     *cdf_index = 0;
   else if ((prev_ul == prev_u) && (prev_ul == prev_l))
     *cdf_index = 2;