More degenerate flow graphs breaking if_convert.

[Jeffrey A Law]

  In message < 20000612134514.A20581@cygnus.com >you write:
  > As far as I can tell, Morgan's algorithm relies on having the
  > program in SSA form.  Or at least a complete use-def graph.
Yea, I hadn't thought of that.  Sigh.

  > I'm not sure that we're ready for such a radical change.  I
  > think we'd be better off in the short term iterating between
  > calculate_global_regs_live and cleanup_cfg.
Ouch.  I'd think that could get potentially expensive.  Though until
we're ready to go with SSA until late in the compiler, it may be our
only reasonable choice.

It may also require we beef up cleanup_cfg.

jeff