ME_SSA isn't ready to be used in the closure converter yet

Some work needs to be done in the Ssa functor so that a multi-entry Ssa
functor can be exported.  Until then, hold off on moving the closure
converter to the multi-entry ssa.
Implement the SSA dominator forest

Since functions no longer have a single root entry point, we need to
have a dominator tree for each entry.  The dominator forest is built by
creating a fake root node that points to each entry point, building a
dominator tree, then throwing the fake root node away.