nir/cf: Conditionally do block_add_normal_succs() in unlink_jump();

nir/cf: Conditionally do block_add_normal_succs() in unlink_jump();

There is a bug where we mess up predecessors/successors due to the
ordering of unlinking/recreating edges/adding fake edges.  In order to
fix that, I need everything in one routine.

However, calling block_add_normal_succs() isn't safe from
cleanup_cf_node() - it would crash trying to insert phi undefs.
So unfortunately I need to add a parameter.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>