Fixes to thread chunk computation, I hope.
Fixes to thread chunk computation, I hope.

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