Optimize DateTime constructors (DateToTicks and IsLeapYear)

[EgorBo]

Just a nano-optimization to make some of the DateTime constructors a bit faster, e.g.:

[Benchmark]
public DateTime CreateDate_const()
{
    return new DateTime(2021, 5, 20);
}

[Benchmark]
[Arguments(2021, 5, 20)]
public DateTime CreateDate_var(int y, int m, int d)
{
    return new DateTime(y, m, d); // and other constructors with DateToTicks/IsLeapYear checks inside
}

/*

            |           Method |      Mean |
            |----------------- |----------:|
     master | CreateDate_const | 0.3712 ns |
     PR     | CreateDate_const | 0.2356 ns |
     master |   CreateDate_var | 2.6116 ns |
     PR     |   CreateDate_var | 2.1922 ns |

*/

Codegen diff for CreateDate_var: https://www.diffchecker.com/BOTFFqql (336 bytes -> 286 bytes), for arm64: https://www.diffchecker.com/H8xUCOtO (448 bytes -> 400 bytes)

Feel free to close if you think the hacks aren't worth it 🙂

[Dotnet-GitSync-Bot]

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[EgorBo]

https://godbolt.org/z/8qdPEK

[EgorBo]

y is int but always is in the [1..9999] range so can be treated as unsigned (X /u C is faster than X / C) sharplab.io

[jkotas]

A similar optimization is part of #46245

[jkotas]

Yes, we should fix the JIT instead of sprinkling manually generated magic div and mod everywhere. I had the same comment in #45479 (comment)

[EgorBo]

ah didn't notice that pr 🙂