We have a smattering of tasks such as build-signing, upload-symbols, etc which we typically only want to run when their associated build is explicitly scheduled.

E.g, if we optimize the linux64/opt away on autoland, we don't want linux64/opt-upload-symbols being the cause of bringing it back. Historically we've gotten around this by copying the dependencies' optimization over to the current task, e.g:
https://searchfox.org/mozilla-central/rev/50cb0892948fb4291b9a6b1b30122100ec7d4ef2/taskcluster/taskgraph/transforms/upload_symbols.py#78

Though this doesn't work if the optimization specified on the dependency relies on task specific metadata (like the label). This is specifically an issue with the bugbug based optimizers (but can theoretically happen with other ones as well).

Tom had started work on an optimizer that handles this properly, and optimizes a task away if its dependencies were also optimized over in:
https://phabricator.services.mozilla.com/D80508

I'm going to take it over and finish it off.

I got tripped up by the docs for Graph.visit_preorder/visit_postorder which
talks about nodes linking to and from other nodes, but makes no mention of
which direction the linking happens practically (in my mind I thought "linking
to" meant going from task -> dependent).

This blurb would have helped me catch my mistake.

This is needed as future revisions will refactor the optimization process to be
two pass. So we can't be sure if a task is optimized or not until the very end.

Depends on D87771

This test takes a long time and is annoying when it's included by default.

Depends on D87772

Depends on D80508

Depends on D87775

https://hg.mozilla.org/mozilla-central/rev/7f1a6c8793e9
https://hg.mozilla.org/mozilla-central/rev/a6752cee069c
https://hg.mozilla.org/mozilla-central/rev/11f1c7a2bfe1

This removes boiler plate around adding new test cases.

Depends on D88482

Depends on D88483

https://hg.mozilla.org/mozilla-central/rev/f7912b1443a5
https://hg.mozilla.org/mozilla-central/rev/e1c247e7155a
https://hg.mozilla.org/mozilla-central/rev/e57bccb01008
https://hg.mozilla.org/mozilla-central/rev/3fa9081e76a9
https://hg.mozilla.org/mozilla-central/rev/c132a4b8fa7a