Optimised SiStripMonitorTracker::AllClusters function

Minor optimization from this
https://vsimetka.web.cern.ch/vsimetka/cgi-bin/igprof-navigator/20.0_old/3/pp/200
to this
https://vsimetka.web.cern.ch/vsimetka/cgi-bin/igprof-navigator/20.0/3/pp/713