Warn if notnull and nullcheck

[JohanLarsson]

private static bool IsReferenceTypeCreation(ObjectCreationExpressionSyntax objectCreation, SyntaxNodeAnalysisContext context)
{
    if (objectCreation == null)
    {
        return false;
    }

    var type = context.SemanticModel.GetTypeInfoSafe(objectCreation, context.CancellationToken)
                      .Type;
    if (type.IsValueType)
    {
        return false;
    }

    return true;
}

In private methods