Add limited size/concat hoisting.
Add limited size/concat hoisting.

I'll need to give the hoister the ability to add entirely new bindings
for this to get better.  So that's next.

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