Optimize typeof(T1).IsAssignableFrom(typeof(T2))

[jkotas]

Optimize typeof(T1).IsAssignableFrom(typeof(T2)) to true/false. E.g.

typeof(IEnumerable<object>).IsAssignableFrom(typeof(string[]));// to `true`

compareTypesForCast seems does everything I need for IsAssignableFrom: handles COMObjects, __Canon, covariance/contravariance, etc. The only thing - it gives up on Nullable<>.

Contributes to https://github.com/dotnet/coreclr/issues/2591

[jkotas]

Reverting the revert.

cc @AndyAyersMS@EgorBo

[jkotas]

Failures are known issues.