Vectorize a few more conversions in TensorPrimitives.ConvertXx

[stephentoub]

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Numerics.Tensors;

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

[MemoryDiagnoser]
public class Tests
{
    private byte[] _bytes;
    private float[] _floats;

    [GlobalSetup]
    public void Setup()
    {
        _bytes = new byte[1024];
        new Random(42).NextBytes(_bytes);
        _floats = new float[_bytes.Length];
    }

    [Benchmark]
    public void Convert() => TensorPrimitives.ConvertTruncating<byte, float>(_bytes, _floats);
}

On my laptop with AVX512...

Before:

Method	Mean
Convert	1.429 us

After:

Method	Mean
Convert	46.59 ns

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