ir3: Make predecessors an array

ir3: Make predecessors an array

We need a stable order in order to create phi instructions. In the
future we can make this more sophisticated in order to make manipulating
the CFG easier, but for now that only happens after RA, so we won't have
to worry about it.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10591>