Fix occurrence checking of checkArg.
Fix occurrence checking of checkArg.

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