Investigate deoptimization diagnostics tooling

[DanielRosenwasser]

We've been diving deep into perf. One place we find tooling lacking is in identifying deoptimizations in the VM. @rbuckton has had some neat experiments which we might be able to leverage to identify polymorphic/megamorphic usage sites. Work closely with @amcasey here to get a sense of what is and isn't useful.

Also, identify some baselines within the tooling to help drive meaningful goals for users. For example, a use-site might have 500 different object shapes passing through it - is it meaningfully faster to reduce that to 200?

Ideally, this would be usable by the broader team and partner teams who also write CPU-bound Node.js code.

[Kingwl]

May I request that be investigation public (Or already public but in somewhere)?
Very interested in this stuff.

[canonic-epicure]

There's already a tool for this: https://www.npmjs.com/package/deoptigate

Its very useful, shows all places where v8 switches to megamorphic IC.