JIT: Remove a TODO comment

[jakobbotsch]

This case is legitimate. A reduced repro looks like:

public class Program
{
    public static async Task Main(string[] args)
    {
        Program p = GetP();
        int x = 0;
        await Aaa();
        if (args.Length > 1)
        {
            x = 456;
            await p.Foo(ref x);
        }
        else
        {
            await p.Bar();
        }
        Console.WriteLine(x);
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static Program GetP() => new Program();

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static async Task Aaa()
    {
    }

    public virtual Task Foo(ref int x)
    {
        return Task.CompletedTask;
    }

    public virtual Task Bar()
    {
        return Task.CompletedTask;
    }
}

In this case we need to save x only in the first path even though x is live in both cases. The layout check is needed to catch this case.
Note that ref x is necessary to the Foo call, but only because otherwise the previous save-set check filters the case.

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[Copilot]

Pull request overview

This PR updates the tail-merging/reuse check for async suspension points in AsyncTransformation::IsReusableSuspension by removing an open-ended TODO and replacing it with an explanatory comment describing why a final continuation layout equality check is still required.

Changes:

• Replace a TODO about unexpected liveness mismatches with a concrete explanation of an observed mismatch case.
• Keep the existing final ContinuationLayoutBuilder::Equals guard, now documented as the catch-all for remaining save/layout disagreements.