Handle existential shapes in allocator.
Handle existential shapes in allocator.

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