Fix 21765 - Assignment-as-condition error with checkaction=context

[MoonlightSentinel]

Don't eagerly check toBoolean if op is nested in another expression.
This still catches assert((a = 1)) but prevents invalid errors for
assignments in assert((a = 1) == 1).

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21765	normal	Assignment-as-condition error with checkaction=context

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#12307"

[MoonlightSentinel]

Thanks @Geod24