RFR(M): 8223051: support loops with long (64b) trip counts

[Roland Westrelin]

> I’m not sure what you mean?  The original incr is an AddL,
> since we are transforming a long loop.  The AddI goes
> somewhere else in the transformed code.

I was confusing myself. Ignore that comment.

> Not any more.  Let’s just make sure the transform gets exercised, OK?

"make sure the transform gets exercised" = properly stress tested?
Tobias listed StressLongCountedLoop he used in an other email.

> After the P.S. is an amended chunk of pseudocode showing how it works.
> I created it by labeling the various expressions in the example loop
> with the names used in is_long_counted_loop, and then I stepped
> through is_long_counted_loop and edited the pseudocode to
> reflect each step.  If you agree I did it correctly, and that it helps
> explain the code, you could place it as a comment at bottom, just
> before the final peel.  Otherwise, we can just leave it here FTR.
>
> I do have this specific request:  Please replace the pseudocode
> at the top (already in the webrev) with the following corrected
> pseudocode.  It uses names more consistent with the actual C++
> code and corresponds more accurately to the transformed IR.

Ok. Let me do that.

Roland.