Better type errors for Rotate in core language.
Better type errors for Rotate in core language.

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