Hi,

When grep uses regex, it splits a pattern with multiple lines by
newline character.  Compilation and executution run for each fragment. 
That causes slowdown.  By this change, each fragment is divided into
groups by whether the fragment includes back reference in a pattern or
not. a frgment which includes back reference constitutes group, and all
frgments which include back reference also constitute a group.

This change extremely speeds-up following case.

- before

    $ yes 00000000000000000000000000000000000000000x | head -10 >in
    $ time -p env LC_ALL=C src/grep -f pat in
    real 1.94
    user 1.70
    sys 0.24

    $ yes 00000000000000000000000000000000000000000x | head -100 >in
    $ time -p env LC_ALL=C src/grep -f pat in
    real 13.04
    user 12.78
    sys 0.25

- after

    $ yes 00000000000000000000000000000000000000000x | head -10 >in
    $ time -p env LC_ALL=C src/grep -f pat in
    real 0.75
    user 0.56
    sys 0.17

    $ yes 00000000000000000000000000000000000000000x | head -100 >in
    $ time -p env LC_ALL=C src/grep -f pat in
    real 0.74
    user 0.57
    sys 0.16

Thanks,
Norihiro