fix(es/minifier): Check assign target before merge assign cond

[Austaras]

Description:

BREAKING CHANGE:

Related issue (if exists):

[changeset-bot]

🦋 Changeset detected

Latest commit: 0a90c1b

The changes in this PR will be included in the next version bump.

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[codspeed-hq]

CodSpeed Performance Report

Merging #10020 will degrade performances by 6.14%

_{Comparing Austaras:main (0a90c1b) with main (7d7319f)}

Summary

⚡ 3 improvements
❌ 2 regressions
✅ 191 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	es/full/minify/libraries/jquery	162.9 ms	170.3 ms	-4.35%
⚡	es/full/minify/libraries/lodash	188.2 ms	179.9 ms	+4.6%
⚡	es/full/minify/libraries/vue	235.8 ms	226.1 ms	+4.27%
⚡	es/minifier/libs/jquery	135.4 ms	128.1 ms	+5.69%
❌	es/minifier/libs/lodash	145.2 ms	154.7 ms	-6.14%