Ensure that split dimensions are Int32s.
Ensure that split dimensions are Int32s.

Fixes #242.

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