Rework branch handling in inliner

[limepoutine]

This eliminates several special cases in the inliner and allows returning under one level of if statement. This work will further allow loops with continue and break to be inlined.

In short, this turns

{
    if (cond)
        return exp1;
    ...
    return exp2;
}

into

{
    if (cond)
        return exp1;
    else
    {
        ...
        return exp2;
    }
}

[dlang-bot]

Thanks for your pull request and interest in making D better, @limepoutine! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

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Testing this PR locally

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dub run digger -- build "master + dmd#22483"