perf(transformer): reuse same UID for _this in arrow function transform

[overlookmotel]

Re-use same _this UID for every scope, instead of creating a new one each time - _this, _this2 etc. It's not necessary for each var to have a different name, as only one this can be in scope anywhere.

Conformance fails because some of Babel's test cases rely on every _this var to be named differently, so we can't merge this, but submitting PR just to see the effect on benchmarks.

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[overlookmotel]

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• perf(transformer): reuse same UID for _this in arrow function transform #5840 👈
• feat(traverse): generate_uid_name method #5839 : 1 other dependent PR (#5841 )
• refactor(transformer): remove repeat code in arrow function transform #5837
• refactor(transformer): deref SymbolId immediately #5836
• perf(transformer): make branch more predictable in arrow function transform #5833
• refactor(transformer): reorder methods in arrow function transform #5830
• fix(transformer): fix stacks in arrow function transform #5828
• refactor(transformer): rename vars in arrow function transform #5827
• refactor(transformer): shorten code in arrow functions transform #5826
• fix(transformer): remove AstBuilder::copy from arrow functions transform #5825
• main

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[codspeed-hq]

CodSpeed Performance Report

Merging #5840 will not alter performance

_{Comparing 09-17-perf_transformer_reuse_same_uid_for__this_in_arrow_function_transform (9778a05) with 09-17-feat_traverse_generate_uid_name_method (82e8778)}

Summary

✅ 29 untouched benchmarks

[overlookmotel]

Interesting. generate_uid is now cheap enough that this doesn't make any noticeable difference on benchmarks. Or maybe our benchmarks don't include enough arrow functions containing this for the code to be exercised.