Improve ComparerHelper.CreateDefaultEqualityComparer

[hanachiru]

Summary

This PR changes the nullable underlying type retrieval from GetGenericArguments() to GetNullableUnderlyingType() in ComparerHelpers.CreateDefaultEqualityComparer.
This change avoids allocating a generic argument array when creating an equality comparer for a closed Nullable<T> and uses the API introduced in #126905.

Benchmark

| Method                        | Job        | Toolchain | type                                | Mean       | Error      | StdDev     | Median     | Min        | Max        | Ratio | RatioSD | Gen0   | Allocated | Alloc Ratio |
|------------------------------ |----------- |---------- |------------------------------------ |-----------:|-----------:|-----------:|-----------:|-----------:|-----------:|------:|--------:|-------:|----------:|------------:|
| CreateDefaultEqualityComparer | Job-HCBSPO | /main     | System.Int32                        | 130.313 ns |  3.5073 ns |  4.0390 ns | 129.996 ns | 126.479 ns | 144.246 ns |  1.00 |    0.00 | 0.0104 |      88 B |        1.00 |
| CreateDefaultEqualityComparer | Job-QOJBTO | /pr       | System.Int32                        | 128.015 ns |  0.8196 ns |  0.7666 ns | 128.138 ns | 126.395 ns | 129.130 ns |  0.98 |    0.03 | 0.0103 |      88 B |        1.00 |
|                               |            |           |                                     |            |            |            |            |            |            |       |         |        |           |             |
| CreateDefaultEqualityComparer | Job-HCBSPO | /main     | System.Int64                        | 129.985 ns |  2.5982 ns |  2.7801 ns | 128.916 ns | 127.263 ns | 137.733 ns |  1.00 |    0.00 | 0.0103 |      88 B |        1.00 |
| CreateDefaultEqualityComparer | Job-QOJBTO | /pr       | System.Int64                        | 130.928 ns |  3.6140 ns |  4.1619 ns | 129.683 ns | 127.794 ns | 147.174 ns |  1.01 |    0.04 | 0.0104 |      88 B |        1.00 |
|                               |            |           |                                     |            |            |            |            |            |            |       |         |        |           |             |
| CreateDefaultEqualityComparer | Job-HCBSPO | /main     | System.Nullable`1[System.DateTime]  | 161.345 ns | 11.2124 ns | 12.9122 ns | 154.015 ns | 152.674 ns | 197.246 ns |  1.00 |    0.00 | 0.0141 |     120 B |        1.00 |
| CreateDefaultEqualityComparer | Job-QOJBTO | /pr       | System.Nullable`1[System.DateTime]  | 132.521 ns |  6.1224 ns |  7.0506 ns | 130.353 ns | 128.537 ns | 157.255 ns |  0.83 |    0.07 | 0.0104 |      88 B |        0.73 |
|                               |            |           |                                     |            |            |            |            |            |            |       |         |        |           |             |
| CreateDefaultEqualityComparer | Job-HCBSPO | /main     | System.Nullable`1[System.DayOfWeek] | 153.169 ns |  1.5292 ns |  1.4304 ns | 153.055 ns | 151.425 ns | 157.249 ns |  1.00 |    0.00 | 0.0140 |     120 B |        1.00 |
| CreateDefaultEqualityComparer | Job-QOJBTO | /pr       | System.Nullable`1[System.DayOfWeek] | 127.887 ns |  0.4688 ns |  0.4385 ns | 127.849 ns | 127.047 ns | 128.805 ns |  0.84 |    0.01 | 0.0104 |      88 B |        0.73 |
|                               |            |           |                                     |            |            |            |            |            |            |       |         |        |           |             |
| CreateDefaultEqualityComparer | Job-HCBSPO | /main     | System.Nullable`1[System.Int32]     | 156.321 ns |  1.7346 ns |  1.6225 ns | 155.847 ns | 154.560 ns | 160.061 ns |  1.00 |    0.00 | 0.0137 |     120 B |        1.00 |
| CreateDefaultEqualityComparer | Job-QOJBTO | /pr       | System.Nullable`1[System.Int32]     | 130.704 ns |  1.0851 ns |  1.0150 ns | 130.424 ns | 129.437 ns | 133.321 ns |  0.84 |    0.01 | 0.0103 |      88 B |        0.73 |
|                               |            |           |                                     |            |            |            |            |            |            |       |         |        |           |             |
| CreateDefaultEqualityComparer | Job-HCBSPO | /main     | System.Nullable`1[System.Int64]     | 160.051 ns |  9.1930 ns | 10.5867 ns | 155.247 ns | 152.374 ns | 192.656 ns |  1.00 |    0.00 | 0.0137 |     120 B |        1.00 |
| CreateDefaultEqualityComparer | Job-QOJBTO | /pr       | System.Nullable`1[System.Int64]     | 131.059 ns |  2.7503 ns |  3.1673 ns | 130.331 ns | 127.530 ns | 137.181 ns |  0.82 |    0.05 | 0.0102 |      88 B |        0.73 |
|                               |            |           |                                     |            |            |            |            |            |            |       |         |        |           |             |
| CreateDefaultEqualityComparer | Job-HCBSPO | /main     | System.String                       |   3.759 ns |  0.1103 ns |  0.1270 ns |   3.779 ns |   3.500 ns |   4.077 ns |  1.00 |    0.00 | 0.0029 |      24 B |        1.00 |
| CreateDefaultEqualityComparer | Job-QOJBTO | /pr       | System.String                       |   3.800 ns |  0.0976 ns |  0.1124 ns |   3.781 ns |   3.637 ns |   4.148 ns |  1.01 |    0.04 | 0.0029 |      24 B |        1.00 |

[BenchmarkCategory(Categories.Libraries)]
public class Perf_EqualityComparer
{
    private static readonly Func<Type, object> s_createDefaultEqualityComparer =
        (Func<Type, object>)typeof(EqualityComparer<>).Assembly
            .GetType("System.Collections.Generic.ComparerHelpers", throwOnError: true)!
            .GetMethod(
                "CreateDefaultEqualityComparer",
                BindingFlags.NonPublic | BindingFlags.Static)!
            .CreateDelegate(typeof(Func<Type, object>));

    public static IEnumerable<Type> Arguments()
    { 
        yield return typeof(int);
        yield return typeof(long);
        yield return typeof(string);
        yield return typeof(int?);
        yield return typeof(long?);
        yield return typeof(DateTime?);
        yield return typeof(DayOfWeek?);
    }

    [Benchmark]
    [ArgumentsSource(nameof(Arguments))]
    [MemoryRandomization]
    public object CreateDefaultEqualityComparer(Type type)
    {
        return s_createDefaultEqualityComparer(type);
    }
}

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[jkotas]

Thanks