Enable egraph-based optimization by default.

[cfallin]

This PR follows up on #5382 and #5391, which rebuilt the egraph-based optimization framework to be more performant, by enabling it by default.

Based on performance results in #5382 (my measurements on SpiderMonkey and bjorn3's independent confirmation with cg_clif), it seems that this is reasonable to enable. Now that we have been fuzzing compiler configurations with egraph opts (#5388) for 6 weeks, having fixed a few fuzzbugs that came up (#5409, #5420, #5438) and subsequently received no further reports from OSS-Fuzz, I believe it is stable enough to rely on.

This PR enables use_egraphs, and also normalizes its meaning: previously it forced optimization (it basically meant "turn on the egraph optimization machinery"), now it runs egraph opts if the opt level indicates (it means "use egraphs to optimize if we are going to optimize"). The conditionals in the top-level pass driver are a little subtle, but will get simpler once we can remove the non-egraph path (which we plan to do eventually!).

Fixes #5181

[cfallin]

Hmm, some issues caused by my reimplementation of #5534 (GVN + idempotency) for egraphs; switching this to draft for now.

[cfallin]

Pushed updates, but I'll factor out the idempotent-GVN updates for egraphs into a separate PR tomorrow then rebase this.

[cfallin]

Rebased on the idempotency fix; should be ready to review now!

[alexcrichton]

I think one final location to touch doc-wise would be here too perhaps?

[cfallin]

I think one final location to touch doc-wise would be here too perhaps?

Ah, yes, updated!

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[fitzgen]

🎉

[cfallin]

In an extremely amusing twist, adding the deprecation warning for the cranelift_use_egraphs config option makes our fuzz target build throw a warning (because it can configure Cranelift not to use egraphs), and this makes our no-warnings build fail. I'm going to just remove the deprecation warning for now to get this in and am happy to talk about ways to warn while still using it internally in a followup PR.

[fitzgen]

In an extremely amusing twist, adding the deprecation warning for the cranelift_use_egraphs config option makes our fuzz target build throw a warning (because it can configure Cranelift not to use egraphs), and this makes our no-warnings build fail. I'm going to just remove the deprecation warning for now to get this in and am happy to talk about ways to warn while still using it internally in a followup PR.

You can #[allow(deprecated)] on the specific function that does the deprecated call in the fuzzing infra.