Let register allocation handle unused TNs due to constant folding
Type-directed constant folding can leave lambda-vars that are neither dead
nor read from or written to. Ideally, it seems like we should make sure to
transform those into REF to CONSTANTs, but the optimisation doesn't seem
guaranteed to fire. It looks like the TN could simply not be allocated
in ASSIGN-LAMBDA-VAR-TNS, but I'm not sure how to test for that situation
ahead of time yet.