when we see a unaryoperator, unless it's one of a small set, we can
know (mostly) that the field will not be written.
there is still a small risk of false+ with code taking references
via conditional expressions.

Change-Id: I96fa808067576a50e5eaf425338e225b4e0bdd4e
Reviewed-on: https://gerrit.libreoffice.org/53263Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>