fix(minifier): improve remove unused variable declaration

[Boshen]

In script mode:

var x;

must be preserved.

fixes #12341

I took the opportunity to refactor these test cases, and also apply "remove unused assignment expression" to all expressions instead of lone assignment expression.

function foo() {
  var x = 1;
  return x = 2;
}

[Boshen]

• fix(minifier): improve remove unused variable declaration #12351 👈 (View in Graphite)
• main

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #12351 will not alter performance

_{Comparing 07-17-fix_minifier_keep_top_level_variable_declarations_in_script_mode (8777839) with main (a55b970)}

Summary

✅ 34 untouched benchmarks

[graphite-app]

Merge activity

• Jul 17, 12:44 PM UTC: Boshen added this pull request to the Graphite merge queue.
• Jul 17, 12:53 PM UTC: Merged by the Graphite merge queue.