RFC -- update_equiv_regs and friends

[Jeffrey A Law]

  In message < 200102281309.f1SD9kI16446@phal.cambridge.redhat.com >you write:
  > > My time is very limited, so I can't take on any large scale development
  > > work to solve this problem.  Thus, I'm thinking seriously about A, C, D,
  > > E & f.
  > 
  > I vote for G: prune the equivalences while they are made in a
  > machine-dependent manner.  That can then also takew care of situations
  > where a constant is more expensive to rematerialize on a particular target
  > than it is in general.
  > 
  > Something like:
I really dislike that approach.  It's leaving too much for the backends to
do.  We really should be able to generically prune bogus equivalences
instead of forcing the backend to do so.

jeff