Add -Zautodiff_post_passes flag to limit which llvm passes to run after enzyme to make autodiff tests more robust

[sgasho]

• Add -Zautodiff_post_passes flag to limit which llvm passes to run after enzyme to make autodiff tests more robust
• Change some llvm ir check in codegen_llvm/autodiff tests

r? @ZuseZ4

[sgasho]

I tested this PR still works after enzyme submodule update: #157716

[ZuseZ4]

I think I've seen some of these changes before, so my concern is that either we continue to make the tests increasingly weaker, or we'll keep swapping back and forth between different outputs.
In the past, I added the noPostOpt flag, which made it a little better, but since the output still changed, we should find a more robust solution before putting it in CI.
Can you have a look at the -C passes=val -- a list of extra LLVM passes to run (space separated) flag, as well as some of the enzyme tests which run extra opts after enzyme?
https://github.com/EnzymeAD/Enzyme/blob/c96508349d9f121957292308c646b4a1fd53c507/enzyme/test/Enzyme/ReverseMode/add.ll#L2

We probably want to remove -Zautodiff=NoPostopt and replace it with -Zautodiff=Passes=... or something like that.
There is a -Zoffload=Host=path flag, which is also nested as an example.
The idea is that not setting the flag would result in the current behavior (full O3 after Enzyme), setting it to "" would be equal to the current NoPostOpt flag (which can then be removed) and by setting anything else we can specify the actual passes to run, similar to Enzyme.

[sgasho]

Thanks!
I added the autodiff_post_passes flag. I thought it would be convenient to pass LLVM passes in the same format LLVM expects, comma separated. If we add a nested option like -Zautodiff=Passes=..., we would need special parsing or escaping, since -Zautodiff options are also separated by commas.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[ZuseZ4]

lgtm, but can you clean up the story a bit? Maybe make one commit first to introduce the infra, and then another one to update the test.
@bors delegate
with updated history and when ci is green.

[rust-bors]

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[sgasho]

@bors r=ZuseZ4

[rust-bors]

📌 Commit 2a9e45d has been approved by ZuseZ4

It is now in the queue for this repository.

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