optimize DateTimeOffset.UtcNow by removing redundant verification

[adamsitnik]

cuts 10% of the time on Windows and contributes to the Caching TE benchmark

[Benchmark]
public DateTimeOffset GetUtcNow() => DateTimeOffset.UtcNow;

BenchmarkDotNet=v0.12.1.1466-nightly, OS=Windows 10.0.18363.1198 (1909/November2019Update/19H2)
Intel Xeon CPU E5-1650 v4 3.60GHz, 1 CPU, 12 logical and 6 physical cores
.NET SDK=5.0.100-rc.2.20479.15
  [Host]     : .NET 5.0.0 (5.0.20.47505), X64 RyuJIT
  Job-MJJALX : .NET 6.0.0 (42.42.42.42424), X64 RyuJIT
  Job-NASKJQ : .NET 6.0.0 (42.42.42.42424), X64 RyuJIT

Method	Toolchain	Mean	Ratio
GetUtcNow	\after\CoreRun.exe	68.91 ns	0.90
GetUtcNow	\before\CoreRun.exe	76.34 ns	1.00

the Caching benchmark shows +4.5% RPS improvement on Windows:

load	before	after
CPU Usage (%)	34	35
Cores usage (%)	957	978
Working Set (MB)	48	48
Build Time (ms)	7,922	4,230
Start Time (ms)	0	0
Published Size (KB)	76,401	76,401
First Request (ms)	85	83
Requests/sec	286,622	299,359
Requests	4,327,835	4,520,311
Mean latency (ms)	2.17	1.63
Max latency (ms)	218.08	250.79
Bad responses	0	0
Socket errors	0	0
Read throughput (MB/s)	901.12	942.08
Latency 50th (ms)	0.88	0.84
Latency 75th (ms)	0.93	0.88
Latency 90th (ms)	0.98	0.93
Latency 99th (ms)	52.50	32.75

[Dotnet-GitSync-Bot]

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[stephentoub]

Thanks.

[benaadams]

Need to change expected exception type in test System.Tests.DateTimeOffsetTests.Ctor_Long_TimeSpan_Invalid from ArgumentException to ArgumentOutOfRangeException

[adamsitnik]

Need to change expected exception type in test

thanks! I could not find the failure logs due to #45357 I was not sure about changing the thrown exception (it would be a breaking change?) so I've changed the order of passed arguments.

[benaadams]

it would be a breaking change?

It's a more derived exception so would be ok; though the test was doing an exact match which is why it broke