ApfV4GeneratorBase: add self() helper func

will be used in places where we need to do nothing (like 'R0 += 0')

(this hides the 'ugly' cast to (Type) in one function)

Tested: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I3ec6db84d2d2f7a14b4edc649df5723598405da2