We can set RE_NO_SUB for calling regex only to check syntax. It brings
performance gains in cases to have a lot of enormous epsilon nodes.
$ printf '(%02d)\n' | sed 's/0/|/g' >pat
(before)
$ time -p env LC_ALL=C src/grep -Ef pat /dev/null
real 6.15
user 4.62
sys 1.52
(after)
$ time -p env
Codes described in attached patch in this mail is dead.
These codes make sense when a maltibyte character is returned in case
conversion for single byte character, but I believe that their cases are
removed in commit 016e590a8198009bce0e1078f6d4c7e037e2df3c.
From e62e727ac271b1cecf5542eebd07eb8501
