extracts code from the innermost part of fairly hot loops

And add a GetIndexFromData method to make the call sites a little easier
to read.

Change-Id: I4ce5c5a687ecdb6982562a0aafce8513d86f9107
Reviewed-on: https://gerrit.libreoffice.org/49337Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>