More backreference performance improvements [take 2]

[Bonzini]

Isamu Hasegawa wrote:

>     I haven't tested all of your patches
>     I tested regex-20-percent-speedup.patch, and
>     regex-25-percent-speedup.patch.  I have no objection for these
patches.

Ok to apply the regex-25-percent-speedup.patch and
regex-more-backref-performance-2.patch (as already suggested by Jack
Howarth)?

Paolo