nir/cf: remove uses of SSA definitions that are being deleted

nir/cf: remove uses of SSA definitions that are being deleted

Unlike calling nir_instr_remove(), calling nir_cf_node_remove() (and
later in the series, the nir_cf_list_delete()) implies that you're
removing instructions that may still have uses, except those
instructions are never executed so any uses will be undefined. When
cleaning up a CF node for deletion, we must clean up any uses of the
deleted instructions by making them point to undef instructions instead.

Signed-off-by: Connor Abbott <connor.w.abbott@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>