Use unoptimized lookup if source is not available

[serenity4]

Fixes #642.

If we hit a CodeInstance that doesn't have inferred code, we now look up its unoptimized (but inferred) source. I think that's the behavior we want, because then we can look into why it wasn't kept (in the case of #642, we can see that lots of constprops were disabled due to LimitedAccuracy, for example). @topolarity feel free to @descend into #642 (comment) and confirm that inspection provides what you would expect to see.

The DCE path for preprocess_ci!(ci::CodeInfo, ...) was apparently unused, I made a few fixes but I believe CallInfos are almost guaranteed to get out of sync which would trigger an assertion error afterwards. Perhaps we should return the IR instead so we can then extract ir.stmts.info? Anyways, I ended up not requiring it and disabling optimization if we have to fall back to an unoptimized lookup, so that seems to remain unused at the moment.

I believe that testing that would require re-enabling the terminal tests, as we'd have to descend into a specific call. I suggest to put that off for now.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (2c33b9a) to head (d70e698).
Report is 63 commits behind head on master.

Files with missing lines	Patch %	Lines
src/CthulhuBase.jl	0.00%	13 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #647   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files           9      11    +2     
  Lines        1556    1572   +16     
======================================
- Misses       1556    1572   +16     

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[vchuravy]

I think we can also just remove preprocess_ci by now, and make Cthulhu a bit more faithful.

[serenity4]

Done in 12e17cc.

What was the motivation to introduce it in the first place? It would seem natural that we'd always want something as faithful as possible for a debugging/introspection tool.

[serenity4]

Ready for final review & merge.

[topolarity]

Nice! It's a bit confusing to see un-optimized IR all of a sudden, but that's pretty easy to notice and the [constprop] Disabled by rettype heuristic (limited accuracy) remarks are very useful.

Looks good to me, thanks @serenity4 !

[topolarity]

(I'm not familiar with Cthulhu internals, so this would still benefit from a review from someone who is)

[serenity4]

@aviatesk a review would be highly appreciated :)

[aviatesk]

Thanks for fixing the issue!

[aviatesk]

TypedSyntax seems to be broken on both nightly and v1.12... We should merge this regardless though.