common block elimination

[Stephen Weeks]

> I didn't bother running the benchmarks -- this should have no effect on
> performance.  But, it did save about .3MB off the size of a self-compile
> executable.

Nice win for such a simple opt.

> This is a bit better, only resetting globals of type exn, but it still
> seems a little long for a 100 line pass.
> 
>             commonBlock starting
>                inferHandlers starting
>                inferHandlers finished in 0.26 + 0.0 (0.0% GC)
>             commonBlock finished in 1.59 + 1.90 (54% GC)

I took a look and only saw one minor thing, which I changed.  This brought the
time down to

	    commonBlock starting
	       inferHandlers starting
	       inferHandlers finished in 0.26 + 0.0 (0.0% GC)
	    commonBlock finished in 1.51 + 0.0 (0.0% GC)

The GC is noise, so this was barely a win.  Anyways, I agree it's a bit strange
that this pass is slower than say cse, but it's not so bad that I'm gonna think
about it.  Oh yeah, one other thing I just saw -- if the order of the newDecs
doesn't matter, you can use an appendRev instead of @.  But that's very minor.