[Bug tree-optimization/84111] [8 Regression] Compile time hog w/ -O2

["matz at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84111

--- Comment #7 from Michael Matz <matz at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #6)
> Sure, but some of the mentioned SSA_NAMEs are registered for update and SCEV
> code is called before that happens.

Yes, sure.  That still doesn't necessarily mean that they have to be generally
aware of this as long as the not-yet-updated SSA form is "reasonably"
reflecting
the original code.  An cycle in SSA name definition quite clearly is
not reasonable, so a better approach is to not generate that situation.

(It might of course be the case that we can't easily avoid creating that
situation, in that case we have to change follow_copies_to_constant()
or friends.  But I don't think we have established that yet).