Faster compilation speed

[Dale Johannesen]

Two points on reference counts.

The compiler I worked on before gcc used RC, and was neither blindingly
fast, nor so slow that anybody complained about it.  Compile speed wasn't
an issue so we never ran any numbers, but my impression was that it was
roughly the same as the gcc in SPEC (2.7 IIRC).  OTOH, we did have quite a
lot of RC bugs, and I think you can expect to also.

And I know this is blindingly obvious, but RC takes an extra field (word,
probably) in each node.  I suspect this is going to eat up a lot of
whatever gain there might be.