new liveness pass

[Stephen Weeks]

> Also, here is the log for a self compile with the new pass.  The self compile
> time is down to 765 seconds.  This is the first time we're under 800!  The funny 
> thing is, the last self compile was at 948 seconds, so the difference can not be 
> explained by this pass alone.  The (unchanged) x86 codegen sped up by 59 seconds!
> My only conjecture is that the backend is leaving around some property lists on
> labels, which is slowing down the codegen.  The new liveness pass doesn't
> leave quite as many around, so maybe that was helping.  I'll look into making
> sure the backend cleans up after itself.

I figured out this discrepancy.  I mistakenly ran the second self compile with a
larger heap, which cut down dramatically on the gc time in the codegen.  I don't
think the backend was leaving around any property lists.

Anyways, the time breakdown for the latest self compile is

generate machine: 240
    x86 code gen: 480
      compile .s:  37
                  ---
            total 757