Compute first byte for forward Progs only.
Compute first byte for forward Progs only.

Change-Id: I1549b85dc830dd1bcee8281516594ad7cb0f4fe0
Reviewed-on: https://code-review.googlesource.com/c/re2/+/56471
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

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