pan/bi: Preserve SSA form from NIR

pan/bi: Preserve SSA form from NIR

Don't call nir_convert_from_ssa, preserve the SSA from NIR. This gets us real
SSA for all frontend passes. The RA becomes a black box that takes SSA in and
outputs register allocated code out. The "broken" SSA form never escapes RA,
which means we can clean up special cases in the rest of the compiler. It also
gets us ready for exploiting SSA form in the RA itself.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17794>