[Patch, fortran] PR35339 Optimize implied do loops in io statements

[Nicolas Koenig]

Hello everyone,

here is a version of the patch that includes a workaround for PR 80960. 
I have also included a separate test case for the failure that Dominique 
detected. The style issues should be fixed.

Regression-tested. OK for trunk?

Nicolas

Changelog:

2017-06-03  Nicolas Koenig  <koenigni@student.ethz.ch>

         PR fortran/35339
         * frontend-passes.c (traverse_io_block): New function.
         (simplify_io_impl_do): New function.
         (optimize_namespace): Invoke gfc_code_walker with
         simplify_io_impl_do.

2017-06-03  Nicolas Koenig  <koenigni@student.ethz.ch>

         PR fortran/35339
         * gfortran.dg/implied_do_io_1.f90: New Test.
         * gfortran.dg/implied_do_io_2.f90: New Test.


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