Fix double-binding of LetWith names.
Fix double-binding of LetWith names.

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