[patch, fortran] Remove workaround introduced because of PR80945

[Thomas Koenig]

Hello world,

after Paul's fix for PR80945, the code in frontend-passes.c meant
to circumvent this bug is no longer needed. The attached patch
removes it, adding a test case which shows that the optimization
is working.

After this, I think we can finally lay PR 35339 to rest.

Regression-tested. OK for trunk?

Regards

	Thomas

2018-02-18  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/35339
         * frontend-passes.c (traverse_io_block): Remove workaround for
         PR 80945.

2018-02-18  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/35339
         * gfortran.dg/implied_do_io_4.f90: New test.
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