[Perf > -20 %] [Mono] Multiple Benchmarks

[naricc]

Multiple benchmarks are showing a significant regression (links to all issues below).

It seems like based on the dates, this is partially caused by #38225 and partially by dd23c81, roughly 50/50 each.

@BrzVlad @DrewScoggins

[Perf -25%] System.Collections.Concurrent.Count.Stack
DrewScoggins/performance-2#2906

[Perf -17%] System.Collections.CreateAddAndClear.ConcurrentQueue
DrewScoggins/performance-2#2905

[Perf -29%] System.Globalization.Tests.StringHash.GetHashCode
DrewScoggins/performance-2#2904

[Perf -26%] System.Threading.Tests.Perf_Timer.SynchronousContention
DrewScoggins/performance-2#2903

Perf -29%] System.Linq.Tests.Perf_Enumerable.Contains_ElementNotFound
DrewScoggins/performance-2#2902

[Perf -29%] System.Linq.Tests.Perf_Enumerable.Contains_ElementNotFound
DrewScoggins/performance-2#2902

[Perf -28%] System.Tests.Perf_Array (2)
DrewScoggins/performance-2#2900

[Perf -31%] BenchmarksGame.FannkuchRedux_9.RunBench
DrewScoggins/performance-2#2899

[Perf -26%] System.Collections.CreateAddAndClear.Span
DrewScoggins/performance-2#2898

[Perf -26%] System.Memory.ReadOnlySpan (2)
DrewScoggins/performance-2#2897

[Perf -24%] System.Collections.Concurrent.Count.Stack
DrewScoggins/performance-2#2896

[Perf -8%] System.MathBenchmarks.Double.CopySign
DrewScoggins/performance-2#2895

[Perf -10%] System.Reflection.Attributes.IsDefinedClassHit
DrewScoggins/performance-2#2894

[Perf -32%] System.Collections.ContainsFalse (3)
DrewScoggins/performance-2#2893

[Perf -9%] System.Text.Json.Tests.Perf_Booleans.WriteBool
DrewScoggins/performance-2#2892

[Perf -13%] System.Tests.Perf_Decimal (2)
DrewScoggins/performance-2#2891

[Perf -32%] System.Collections.ContainsTrue (5)
DrewScoggins/performance-2#2890

[Perf -8%] System.Buffers.Text.Tests.Utf8FormatterTests.FormatterUInt64 
DrewScoggins/performance-2#2889

[Perf -27%] Layout.SearchLoops.LoopReturn
DrewScoggins/performance-2#2886

Related issue in CoreCLR for the thread pool change: #43905

[naricc]

@BrzVlad did an investigation and found that this is because dd23c81 caused an arbitrary perturbation of the register allocator in clang. Other significant changes to the interpreter are coming soon, which will further perturb it, so it doesn't make sense to try to optimize this now. Follow up PR to come.