[patch, Fortran] Fix some string temporaries

[Thomas Koenig]

Hello world,

this fixes PR 71902. The recent fix for PR 71783 introduced a
performance and code size regression a string temporary was created for
the test case when it was not actually needed.

I also took the opportunity of renaming the misnomed temporary
variable.

Regression-tested.

OK for trunk?

Do we actually want to backport this? Technically, it is a regression,
but people are not likely to notice much.

Regards

	Thomas

2016-07-16  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/71902
         * dependency.c (gfc_check_dependency): Use dep_ref.  Handle case
         if identical is true and two array element references differ.
         (gfc_dep_resovler):  Move most of the code to dep_ref.
         (dep_ref):  New function.
         * frontend-passes.c (realloc_string_callback):  Name temporary
         variable "realloc_string".

2016-07-16  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/71902
         * gfortran.dg/dependency_47.f90:  New test.
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