Fix #21105: Is-expressions should not generate unnecessary code for lowerings

[rainers]

treat as if compiling with __traits(compiles)

This also avoids #21066 from running into #21104

[dlang-bot]

Thanks for your pull request, @rainers!

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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#21106"