pan/bi: Clean up after converting to SSA

pan/bi: Clean up after converting to SSA

nir_invalidate_divergence was introduced in aa765c54bdd ("pan/bi: Add divergent
intrinsic lowering pass"), which needed divergence information for a late NIR
pass but not otherwise in the backend. Unfortunately, nir_convert_from_ssa is
less aggressive about coalescing when divergence information makes it look like
the coalescing would make the code worse -- even though that's not actually an
issue on Mali.

Now that we don't use nir_convert_from_ssa, we don't need the hack.

Likewise for the vec.reg hack, which was introduced because the split/collect
cache relies on SSA form.

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