Switch update likely to update every cache line to MERGE.

test/test_compact_mask_index.f90

@@ -124,7 +124,8 @@ CONTAINS
     DO k = strt(3), ends(3)
       DO j = strt(2), ends(2)
         DO i = strt(1), ends(1)
-          IF (mask(i, j, k)) accum_ref(i, j, k) = accum_ref(i, j, k) + 1.0
+          accum_ref(i, j, k) = accum_ref(i, j, k) &
+               + MERGE(1.0, 0.0, mask(i, j, k))
         END DO
       END DO
     END DO