Exit early from pg_comp_crc32c_pmull for small inputs

Exit early from pg_comp_crc32c_pmull for small inputs

The vectorized path in commit fbc57f2bc had a side effect of putting
more branches in the path taken for small inputs. To reduce risk
of regressions, only proceed with the vectorized path if we can
guarantee that the remaining input after the alignment preamble is
greater than 64 bytes. That also allows removing length checks in
the alignment preamble.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/CANWCAZZ48GuLYhJCcTy8TXysjrMVJL6n1n7NP94=iG+t80YKPw@mail.gmail.com