[perf] System.Linq.** performance regressions in Mono AOT x64 microbenchmarks from Preview 5 to Preview 7

[kotlarmilos]

Description

The following performance regressions occurred between the Preview 4 snapshot (May 2nd, 2023) and Preview 7 snapshot (July 19th, 2023).

The configuration used for the benchmarks is Mono AOT with LLVM on x64 ubuntu 18.04.

Name	Baseline Value	Compare Value	% Difference
System.MathBenchmarks.Single.FusedMultiplyAdd	34642.37	36545.44	5.49
System.Linq.Tests.Perf_Enumerable.Sum(input: IEnumerable)	563.21	722.65	28.31
System.Linq.Tests.Perf_Enumerable.SkipHalfTakeHalf(input: IEnumerable)	895.63	951.91	6.28
System.Linq.Tests.Perf_Enumerable.SingleWithPredicate_LastElementMatches(input: List)	721.62	786.92	9.05
System.Linq.Tests.Perf_Enumerable.FirstWithPredicate_LastElementMatches(input: IEnumerable)	651.45	698.17	7.17
System.Linq.Tests.Perf_Enumerable.FirstWithPredicate_LastElementMatches(input: Array)	649.19	738.11	13.7

[kotlarmilos]

@stephentoub Are you aware of any changes that might have potentially caused these regressions? Maybe #88249?

[stephentoub]

Are you aware of any changes that might have potentially caused these regressions?

Not really. I don't think #88249 could be related to any of those; it touched LINQ but in areas I don't think are exercised there.

[kotlarmilos]

Looks like noise in measurements.