nir/dead_cf: Remove dead control flow after infinite loops.

And after discard-only loops. Otherwise we end up with dead code
which confuses nir_repair_ssa into adding a whole bunch of uses
of undefined. However, for derefs, we sometimes always expect to
get a variable instead of undefined.

Fixes dEQP-VK.graphicsfuzz.write-red-in-loop-nest on radv.

Fixes: c832820ce9 "nir/dead_cf: Repair SSA if the pass makes progress"
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1928
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

src/compiler/nir/nir_opt_dead_cf.c

@@ -334,6 +334,13 @@ dead_cf_list(struct exec_list *list, bool *list_ends_in_jump)
          bool dummy;
          progress |= dead_cf_list(&loop->body, &dummy);
 
+         nir_block *next = nir_cf_node_as_block(nir_cf_node_next(cur));
+         if (next->predecessors->entries == 0 &&
+             (!exec_list_is_empty(&next->instr_list) ||
+             !exec_node_is_tail_sentinel(next->cf_node.node.next))) {
+            remove_after_cf_node(cur);
+            return true;
+         }
          break;
       }