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

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.