message: Update reference implementation in INDEXLIST_3LOOP
diff: ///
  /// INDEXLIST_3LOOP kernel reference implementation:
  ///
  +/// for (Index_type i = ibegin; i < iend; ++i ) {
  +/// counts[i] = (x[i] < 0.0) ? 1 : 0;
  +/// }
  +///
  /// Index_type count = 0;
  +/// for (Index_type i = ibegin; i < iend+1; ++i ) {
  +/// Index_type inc = counts[i];
  +/// counts[i] = count;
  +/// count += inc;
  +/// }
  +///
  /// for (Index_type i = ibegin; i < iend; ++i ) {
  -/// if (x[i] < 0.0) { \
  -/// list[count++] = i ; \
  +/// if (counts[i] != counts[i+1]) {
  +/// list[counts[i]] = i;
  /// }
  /// }
  -/// Index_type len = count;
  +///
  +/// Index_type len = counts[iend];
  ///
  #ifndef RAJAPerf_Basic_INDEXLIST_3LOOP_HPP
  