shortcut simple class casts

[WalterBright]

Per the description in https://issues.dlang.org/show_bug.cgi?id=24332 except that it isn't inlined. I want to make some other improvements before trying to inline it.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

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dub run digger -- build "master + dmd#16027"

[WalterBright]

I'm glad I did this as a function first without worrying about inlining it, as it had a couple errors in it.