nir: remove simple dead if detection from nir_opt_dead_cf()

This was probably useful when it was first written, however it
looks to be no longer necessary.

As far as I can tell these days dce is smart enough to remove useless
instructions from if branches. Once this is done
nir_opt_peephole_select() will end up removing the empty if.

Removing this support reduces the dolphin uber shader compilation
time spent in nir_opt_dead_cf() by a little over 7x.

No shader-db changes on i965 or radeonsi.

Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

src/compiler/nir/nir_opt_dead_cf.c

@@ -164,7 +164,7 @@ def_only_used_in_cf_node(nir_ssa_def *def, void *_node)
 }
 
 /*
- * Test if a loop node or if node is dead. Such nodes are dead if:
+ * Test if a loop node is dead. Such nodes are dead if:
  *
  * 1) It has no side effects (i.e. intrinsics which could possibly affect the
  * state of the program aside from producing an SSA value, indicated by a lack
@@ -182,7 +182,7 @@ def_only_used_in_cf_node(nir_ssa_def *def, void *_node)
 static bool
 node_is_dead(nir_cf_node *node)
 {
-   assert(node->type == nir_cf_node_loop || node->type == nir_cf_node_if);
+   assert(node->type == nir_cf_node_loop);
 
    nir_block *after = nir_cf_node_as_block(nir_cf_node_next(node));
 
@@ -239,11 +239,6 @@ dead_cf_block(nir_block *block)
 {
    nir_if *following_if = nir_block_get_following_if(block);
    if (following_if) {
-      if (node_is_dead(&following_if->cf_node)) {
-         nir_cf_node_remove(&following_if->cf_node);
-         return true;
-      }
-
       if (!nir_src_is_const(following_if->condition))
          return false;