nir: Add a better out-of-SSA pass

nir: Add a better out-of-SSA pass

This commit rewrites the out-of-SSA pass to not be nearly as naieve.  It's
based on "Revisiting Out-of-SSA Translation for Correctness, Code Quality,
and Efficiency" by Boissinot et. al.  It should be fairly close to
state-of-the art.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>