[patch, fortran] Fix PR 100227, write with implied DO loop

[Thomas Koenig]

Hello world,

after a bit of an absence, I am now back, at least for some regression
fixing (and for reviewing patches, if that is called for).

So, here's a regression fix to start with.

OK for trunk and affected branches (down to 9)?

Best regards

	Thomas

Do not replace variable op variable in I/O implied DO loop replacement.

This PR came about because index expressions of the form k+k in
implied DO loops in I/O statements were considered for replacement
by array slices.

Fixed by only doing the transformation if the expression is of the
type expr OP contastant.

gcc/fortran/ChangeLog:

         PR fortran/100227
         * frontend-passes.c (traverse_io_block): Adjust test for
	when a variable is eligible for the transformation to
	array slice.

gcc/testsuite/ChangeLog:

         PR fortran/100227
         * gfortran.dg/implied_do_io_7.f90: New test.
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