The very first step towards #83422 - which will move DAG combines to be
processed in topological order.
There is a lot of churn on existing tests that need to be addressed
before this can be switched on globally, this patch gives the ability to
enable it both on a per-target basis, and via a command line option to
assist with testing and triage.
At the moment I'm focusing on addressing the x86 regressions (example in
the patch's basic test coverage) as that's the target I'm most familiar
with and will help with many other targets as well, but there might be
other/simpler targets that would benefit from earlier handling.
207598a827