Revert "Proper scope hygiene in ImpGen."
Revert "Proper scope hygiene in ImpGen."

This reverts commit 542b2169931b9a6c5211e135995d74fcc54f6e17.

Unfortunately the multicore backend is not ready for this.

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