[Bug tree-optimization/90316] [8/9 Regression] large compile time increase in opt / alias stmt walking for Go example

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90316

--- Comment #28 from Richard Biener <rguenth at gcc dot gnu.org> ---
Created attachment 46339
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46339&action=edit
untested patch

This is another patch.  It changes the iteration scheme to RPO order instead of
DOM order to not require iteration unless AVAIL_OUT changes on a backedge
(which usually should not happen for regular PRE insertions...).

Your reduced testcases were all acyclic so this should fix the slowness there.