[patch, fortran] Complete implementation and correction of simplification

[Thomas Koenig]

Hello world,

the attached patch implements simplification for cshift completely.
It also fixes a bug where compile-time simplification was handled
incorrectly for a negative value. For PR 83650 (the wrong
simplification) for the other branches I suspect a quite simple fix will
be possible, which I will address separately.

Regression-tested. OK for trunk?

Regards

	Thomas

2018-01-02  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/45689
         PR fortran/83650
         * simplify.c (gfc_simplify_cshift): Re-implement to allow full
         range of arguments.

2018-01-02  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/45689
         PR fortran/83650
         * gfortran.dg/simplify_cshift_1.f90: Correct erroneous case.
         * gfortran.dg/simplify_cshift_4.f90: New test.

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