[corlib] Optimize System.Nullable<T>.GetValueOrDefault().

[corlib] Optimize System.Nullable<T>.GetValueOrDefault().

Optimize System.Nullable<T>.GetValueOrDefault().

See discussion at:
	http://ericlippert.com/2012/12/20/nullable-micro-optimizations-part-one/

	If a variable of nullable value type is initialized with the
	default constructor then the [has_value] field will be its
	default value, false, and the value field will be default(T).

Since we know that Nullable<T>.value will be the default value when
the default constructor has executed, we can return it directly
instead of checking the has_value field.