...found with (trunk) libc++ on macOS, where the standard library iterator
implementations are apparently sufficiently different to libstdc++ to find more
cases of !(... == ...) vs. ... != ...

Change-Id: Ia3861406aa584c7b1d33f47448190b5cf2e770f7
Reviewed-on: https://gerrit.libreoffice.org/68089
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>