segredomap: all threads in segscan version run Combine and GroupScan
segredomap: all threads in segscan version run Combine and GroupScan

Before, if there were "spare" threads, they would not run Combine or GroupScan,
causing them to not participate in the barriers. Somehow it seemed to work
anyways.

diku-dk/futhark,diku-dk/futhark,HIPERFIT/futhark,HIPERFIT/futhark,ihc/futhark,ihc/futhark,diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,ihc/futhark,HIPERFIT/futhark