Replace DoubleUtil.IsNaN with double.IsNaN

[omariom]

BCL's double.IsNaN got faster long ago. In .NET Core it is a single instruction.

The usage of DoubleUti.IsNaN should be replaced by double.IsNaN.

Codegen difference

Benchmarks

    [SimpleJob(RuntimeMoniker.Net48),
     //SimpleJob(RuntimeMoniker.NetCoreApp31),
     SimpleJob(RuntimeMoniker.NetCoreApp50),
    ]
    [DisassemblyDiagnoser]
    public class IsNaNBenchmarks
    {
        [Benchmark(Baseline = true)]
        [Arguments(123.45)]
        [Arguments(double.NaN)]
        public bool Clr(double value) => double.IsNaN(value);

        [Benchmark()]
        [Arguments(123.45)]
        [Arguments(double.NaN)]
        public bool Wpf(double value) => IsNaN(value);

        public static bool IsNaN(double value)
        {
            NanUnion t = new NanUnion();
            t.DoubleValue = value;

            ulong exp = t.UintValue & 0xfff0000000000000;
            ulong man = t.UintValue & 0x000fffffffffffff;

            return (exp == 0x7ff0000000000000 || exp == 0xfff0000000000000) && (man != 0);
        }

        [StructLayout(LayoutKind.Explicit)]
        private struct NanUnion
        {
            [FieldOffset(0)]
            internal double DoubleValue;
            [FieldOffset(0)]
            internal ulong UintValue;
        }
    }

Results:

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19042
Intel Core i7-8809G CPU 3.10GHz (Kaby Lake G), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=6.0.100-preview.2.21155.3
[Host] : .NET Core 5.0.4 (CoreCLR 5.0.421.11614, CoreFX 5.0.421.11614), X64 RyuJIT
.NET 4.8 : .NET Framework 4.8 (4.8.4341.0), X64 RyuJIT
.NET Core 5.0 : .NET Core 5.0.4 (CoreCLR 5.0.421.11614, CoreFX 5.0.421.11614), X64 RyuJIT

Method	Job	Runtime	value	Mean	Error	StdDev	Median	Ratio	RatioSD	Code Size
Clr	.NET 4.8	.NET 4.8	NaN	1.4431 ns	0.3717 ns	1.0903 ns	0.7239 ns	1.00	0.00	67 B
Wpf	.NET 4.8	.NET 4.8	NaN	1.5641 ns	0.0412 ns	0.0365 ns	1.5708 ns	1.63	0.73	116 B
Clr	.NET Core 5.0	.NET Core 5.0	NaN	0.0281 ns	0.0278 ns	0.0247 ns	0.0231 ns	?	?	14 B
Wpf	.NET Core 5.0	.NET Core 5.0	NaN	1.5333 ns	0.0370 ns	0.0328 ns	1.5350 ns	?	?	106 B
Clr	.NET 4.8	.NET 4.8	123.45	0.6543 ns	0.0348 ns	0.0308 ns	0.6531 ns	1.00	0.00	67 B
Wpf	.NET 4.8	.NET 4.8	123.45	1.8958 ns	0.0343 ns	0.0287 ns	1.9000 ns	2.91	0.14	116 B
Clr	.NET Core 5.0	.NET Core 5.0	123.45	0.0909 ns	0.0276 ns	0.0258 ns	0.0981 ns	1.00	0.00	14 B
Wpf	.NET Core 5.0	.NET Core 5.0	123.45	1.4530 ns	0.0390 ns	0.0346 ns	1.4525 ns	17.92	6.38	106 B

[dipeshmsft]

Closing issue as the corresponding PR has been merged.