Model register usage for write-barriers on x64 more precisely #128720

public sealed class Node { public object? Next; }

public static class Demo
{
    [MethodImpl(MethodImplOptions.NoInlining)]
    public static Vector256<double> Loop(Node[] nodes, object val)
    {
        Vector256<double> acc = Vector256<double>.Zero;
        Vector256<double> one = Vector256.Create(1.0);
        for (int i = 0; i < nodes.Length; i++)
        {
            acc += one;            // loop-carried 256-bit FP accumulator
            nodes[i].Next = val;   // <-- GC-ref store ⇒ CORINFO_HELP_ASSIGN_REF (write barrier)
        }
        return acc;
    }
}