I think things are clear enough to propose a patch.  Thanks for 
everyone's input.

I have added some comments and tweaked Michael's patch to include the 
final BB where we're threading to, in the check.  Without this last 
check, we fail in tree-ssa/cunroll-15.c because the latch becomes 
polluted with PHI nodes.

OK for trunk?
Aldy

commit e735a2cd01485773635bd66c97af21059992d5a7 (HEAD -> 
pending/global-ranges)
Author: Aldy Hernandez <aldyh@redhat.com>
Date:   Thu Sep 9 20:30:28 2021 +0200

     Disable threading through latches until after loop optimizations.

     The motivation for this patch was enabling the use of global ranges in
     the path solver, but this caused certain properties of loops being
     destroyed which made subsequent loop optimizations to fail.
     Consequently, this patch's mail goal is to disable jump threading
     involving the latch until after loop optimizations have run.

     I have added a bit in cfun to indicate when the "loopdone" optimization
     has completed.  This helps the path threader determine when it's 
safe to
     thread through loop structures.  I can adapt the patch if there is an
     alternate way of determining this.

     As can be seen in the test adjustments, we mostly shift the threading
     from the early threaders (ethread, thread[12] to the late threaders
     thread[34]).  I have nuked some of the early notes in the testcases
     that came as part of the jump threader rewrite.  They're mostly noise
     now.

     Note that we could probably relax some other restrictions in
     profitable_path_p when loop optimizations have completed, but it would
     require more testing, and I'm hesitant to touch more things than needed
     at this point.  I have added a reminder to the function to keep this
     in mind.

     Finally, perhaps as a follow-up, we should apply the same 
restrictions to
     the forward threader.  At some point I'd like to combine the cost 
models.

     Tested on x86-64 Linux.

     p.s. There is a thorough discussion involving the limitations of jump
     threading involving loops here:

             https://gcc.gnu.org/pipermail/gcc/2021-September/237247.html