Comments regarding the validity of the Fortran assumptions are welcome!

This patch now uses a 'simple' loop for OpenMP loops with
a constant loop-step size. Before, it only did so for step = ±1.
(Otherwise, a count variable is used from which the original
loop index variable is calculated from.)

For details, see the attached patch or
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107424#c12
(comment 12 + 14 plus the email linked in comment 12).

Comments? Remarks? If there are none, I will relatively soonish
commit the attached patch to mainline, only.

Tobias
-----------------
Siemens Electronic Design Automation GmbH; Anschrift: Arnulfstraße 201, 80634 München; Gesellschaft mit beschränkter Haftung; Geschäftsführer: Thomas Heurung, Frank Thürauf; Sitz der Gesellschaft: München; Registergericht München, HRB 106955