*ping* [patch, fortran, 4.9] Dependency and string length calculation improvements

[Thomas Koenig]

I wrote:

Ping**2?

I'd like to get these patches committed, if possible, to clear up my
trees a little bit :-)

	Thomas
> *ping*
>
> Slightly updated patch below, with a better test case as suggested
> by Dominique.
>
> OK for trunk?
>
> 2013-03-16  Thomas Koenig  <tkoenig@gcc.gnu.org>
>
>          PR fortran/45159
>          * gfortran.h (gfc_dep_difference):  Add prototype.
>          * dependency.c (discard_nops):  New function.
>          (gfc_dep_difference):  New function.
>          (check_section_vs_section):  Use gfc_dep_difference
>          to calculate the difference of starting indices.
>          * trans-expr.c (gfc_conv_substring):  Use
>          gfc_dep_difference to calculate the length of
>          substrings where possible.
>
> 2013-03-16  Thomas Koenig  <tkoenig@gcc.gnu.org>
>
>          PR fortran/45159
>          * gfortran.dg/string_length_2.f90:  New test.
>          * gfortran.dg/dependency_41.f90:  New test.
>