Cleanup QFileIconProviderPrivate::getWinIcon().

No functionality is changed, just made the code more readable
and removed some duplication.

Needed to backport qtbase/46685f755b01288fd53c4483cb97a22c426a57f0.

Code in Qt5 is already clean, no commit necessary there.

Change-Id: I29fe871eeb0b45b619434e7941cc673033366f63
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>