Improve TeddyHelper.RightShift helpers for AVX512

[MihaZupan]

Instead of a PermuteVar8x64x2 + AlignRight, we can do the whole thing with a byte-based permute PermuteVar64x8x2.

public class SearchValuesString
{
    private static readonly SearchValues<string> s_values = SearchValues.Create(["abc", "def", "ghi"], StringComparison.Ordinal);
    private static readonly string s_text = new('\n', 100_000);

    [Benchmark]
    public void ContainsAny() => s_text.AsSpan().ContainsAny(s_values);
}

Method	Toolchain	Mean	Error	Ratio
ContainsAny	main	2.284 us	0.0320 us	1.00
ContainsAny	pr	2.006 us	0.0064 us	0.88

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Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.