improve the read-only check to ignore reads from fields that are guarded
by a boolean check, something like:
   if (field)
       field.foo();
this produces some false positives at the moment because I'm not
correctly handling the else block, but also some useful new dead code.

Change-Id: Id21fa1a56c171d09d979769b978b6eef14e8b695
Reviewed-on: https://gerrit.libreoffice.org/52664Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>