Unroll SpanHelpers.SequenceEqual in a late phase

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checking diffs..

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/azp run runtime-outerloop, runtime-coreclr pgo

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@EgorBot -intel -arm64 -profiler

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

public class Bench
{
    static byte[] Data = "1111222233334444"u8.ToArray();

    [Benchmark]
    public bool Test() => Data.AsSpan().SequenceEqual("1111222233334444"u8);
}