Compute first byte for forward Progs only.

Change-Id: I1549b85dc830dd1bcee8281516594ad7cb0f4fe0
Reviewed-on: https://code-review.googlesource.com/c/re2/+/56471
Reviewed-by: Paul Wankadia <junyer@google.com>
(cherry picked from commit 1956725f5727ca8a739b3af5adf357c89bfe071b)
Reviewed-on: https://code-review.googlesource.com/c/re2/+/56472