fix(minifier): collapse variables when join_vars option is true

[DonIsaac]

The variable collapse AST pass was only collapsing declarations when CompressOptions::join_vars was true, and skipping when false. This does not follow the behavior documented for this option. This PR fixes this.

[DonIsaac]

• test(minifier): add variable collapsing test cases #6227
• fix(minifier): collapse variables when join_vars option is true #6226 👈
• main

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[codspeed-hq]

CodSpeed Performance Report

Merging #6226 will not alter performance

_{Comparing don/10-01-fix_minifier_collapse_variables_when_join_vars_option_is_true (ea896ec) with main (4b42047)}

Summary

✅ 29 untouched benchmarks

[overlookmotel]

I don't know the minifier at all, so I think best leave Boshen to review this.

[7086cmd]

It appears that the test262 breakage is caused by the void x declaration. However, surprisingly, this pass does not address the void x issue at all!

[Boshen]

closed by #6464