nir: Allocate nir_ssa_def::uses/if_uses out of the instruction.

nir: Allocate nir_ssa_def::uses/if_uses out of the instruction.

We can't allocate them out of the nir_ssa_def itself, because it may not
be ralloc'd (for example, nir_dest embeds a nir_ssa_def).

However, allocating them out of the instruction should work.

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