[patch, fortran] Simplify constants which come from parameter arrays

[Thomas König]

Hi Dominique,

> If I am not mistaken, the patch causes:
> 
> FAIL: gfortran.dg/pr71935.f90   -O  (test for excess errors)
> FAIL: gfortran.dg/substr_6.f90   -O0  execution test
> FAIL: gfortran.dg/substr_6.f90   -O1  execution test
> FAIL: gfortran.dg/substr_6.f90   -O2  execution test
> FAIL: gfortran.dg/substr_6.f90   -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions  execution test
> FAIL: gfortran.dg/substr_6.f90   -O3 -g  execution test
> FAIL: gfortran.dg/substr_6.f90   -Os  execution test

These have been resolved now - I have removed the invalid code
from substr_6.f90 (PR85130), and the error is now suppressed
in the attached patch.

Re-regression-tested. OK for trunk?

Regards

	Thomas

2018-03-25  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/51260
         * resolve.c (resolve_variable): Simplify cases where access to a
         parameter array results in a single constant.

2018-03-25  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/51260
         * gfortran.dg/parameter_array_element_3.f90: New test.
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