First()/Last() and xxxOrDefault() variations check for IList, not IReadOnlyList for indexed optimzation

[marcus-sonestedt]

See this line as an example:

referencesource/System.Core/System/Linq/Enumerable.cs

Line 1067 in ec9fa9a

IList<TSource> list = source as IList<TSource>;

It only uses the indexing, which is the only thing IReadonlyList differs on over a collection.