nir/repair_ssa: Repair dominance for unreachable blocks

NIR currently assumes that unreachable blocks are trivially dominated by
everything.  However, when considering well-formed SSA, there is no path
from any block to an unreachable block.  Therefore, we can break any
use-def chains where the use is in an unreachable block.  This removes
any dependencies on code created by uses in unreachable blocks and lets
DCE do a better job of cleaning it up.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit a3268599)