Reduce allocations in ProjectChanges

[kevingosse]

Many methods in Microsoft.CodeAnalysis.ProjectChanges work by building a temporary hashset to compare a list of items between OldProject and NewProject. This came up as a hot spot during a profiling session, for a function that does something like:

var solutionChanges = currentSolution.GetChanges(oldSolution);

foreach (var projectChanges in solutionChanges.GetProjectChanges())
{
    var addedAnalyzerReferences = projectChanges.GetAddedAnalyzerReferences();

    ...
}

If there are no actual changes (which I expect to be the most common case), GetAddedAnalyzerReferences will still create a hashset and go over the whole collection. I believe this can be optimized with a simple check:

    public IEnumerable<AnalyzerReference> GetAddedAnalyzerReferences()
    {
        // Fast-path
        if (NewProject.AnalyzerReferences.Equals(OldProject.AnalyzerReferences))
        {
            yield break;
        }

        // Old logic
        var oldAnalyzerReferences = new HashSet<AnalyzerReference>(OldProject.AnalyzerReferences);
        foreach (var analyzerReference in NewProject.AnalyzerReferences)
        {
            if (!oldAnalyzerReferences.Contains(analyzerReference))
            {
                yield return analyzerReference;
            }
        }
    }

Same thing for all the methods in ProjectChanges that use a similar pattern. It adds a negligible cost when the collections are different, and greatly reduces the cost when the collections are identical.

[CyrusNajmabadi]

Should just be:

=> NewProject.AnalyzerReferences == OldProject.AnalyzerReferences ? [] : ewProject.AnalyzerReferences.Except(OldProject.AnalyzerReferences);

Feel free to create a PR. We can review it asap.