RSSA backend

[Stephen Weeks]

> I like making the array initialization explicit in RSSA, but,
> unfortunately, it doesn't solve the whole problem.  I forgot that the
> control flow to update the frontier based on whether or not numElts is
> zero requires branches (and hence, new labels).

Darn.  This is annnoying also because there is already such a test in
the initialization code.   Someday we're gonna move the frontier
munging code into RSSA I hope, and avoid this problem, but not now.

Another possibility is to simply add POINTER_SIZE to the frontier
uncoditionally.  Then you don't need extra labels.