[Patch, fortran] PR91686 - ICE in gimplify:2554

[Paul Richard Thomas]

Committed as 'obvious' in revision 275681:

2019-09-12  Paul Thomas  <pault@gcc.gnu.org>

    PR fortran/91686
    Backport from mainline
    * trans-expr.c (gfc_trans_assignment_1): Copy and paste section
    handling the rse.pre block from mainline.

2019-09-12  Paul Thomas  <pault@gcc.gnu.org>

    PR fortran/91686
    * gfortran.dg/pr91686.f90 : New test.

The rse.pre block was being added outside the loop so that a temporary
index was being used before it was declared and the loop index was not
defined.

Paul