Fix map-scan fusion.
Fix map-scan fusion.

ihc/futhark,ihc/futhark,mrakgr/futhark,HIPERFIT/futhark,diku-dk/futhark,diku-dk/futhark,ihc/futhark,HIPERFIT/futhark,HIPERFIT/futhark,diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,mrakgr/futhark,CulpaBS/wbBach,mrakgr/futhark