Let register allocation handle unused TNs due to constant folding

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.

 Kludges over lp#729765.