TaskItemSpecFilenameComparer.Compare allocations 0.7% of a solution-wide design-time build

[davkean]

[davkean]

From this trace: dotnet/project-system#2712

[rainersigwald]

https://github.com/Microsoft/msbuild/blob/2d8a4341875e532bdf25a14f208f62a7bb547774/src/Tasks/AssemblyDependency/TaskItemSpecFilenameComparer.cs#L48-L66

Could probably replace those Path.GetFileName with a find-last-slash and a partial string compare.