i965/nir: Lower to registers a bit later.

i965/nir: Lower to registers a bit later.

We can't safely call nir_optimize() with register present, since several
passes called in the loop can't handle registers, and will fail asserts.

Notably, nir_lower_vec_alus() and nir_opt_algebraic() really don't want
registers.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>