Make the NFA execution engine use Prog::EmptyFlags().
Make the NFA execution engine use Prog::EmptyFlags().

Change-Id: Iaab77132da28aaf1b43e45b1c72d2597397462c3
Reviewed-on: https://code-review.googlesource.com/c/35770
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2