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This makes Enum.GetUnderlyingType about 6x faster.

I am going to revert the change for IsEnum. It is behavior change for generic variables. We do not have a test coverage for it. The test coverage needs to be fixed first.

For example, this prints true currently, but it would print false with this change:

Type param = typeof(MyG<>).GetGenericArguments()[0];
Console.WriteLine(param.IsEnum);

class MyG<T> where T: Enum
{
}

And this fails with Internal CLR error. currently:

Type param = typeof(MyG<>).GetGenericArguments()[0];
Console.WriteLine(Enum.GetUnderlyingType(param));

class MyG<T> where T: Enum
{
}

And this fails with Internal CLR error. currently:

Opened #71884

Improvements on win-arm64: dotnet/perf-autofiling-issues#6757

Improvements on ubuntu arm64: dotnet/perf-autofiling-issues#6982