With the recent change to rely on the FSM bits to handle threading 
backedges, there's no way to get into 
record_temporary_equivalences_from_stmts_at_dest after following a backedge.

Thus that argument can be removed and everywhere it was used can be 
simplified assuming its value was false.  That in turn eliminates the 
need for the invalidate method in the const_and_copies class.

There'll be more, but this was one can be easily proved safe and 
correct.  Others need more thought.

Bootstrapped and regression tested on x86_64-linux-gnu.  Installed on 
the trunk.

Jeff