Add the beginnings of better handling of branching in hoister.
Add the beginnings of better handling of branching in hoister.

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