nir/liveness: Consider if uses in nir_ssa_defs_interfere

Fixes: f86902e75d "nir: Add an SSA-based liveness analysis pass"
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3428
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Yevhenii Kharchenko <yevhenii.kharchenko@globallogic.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6824>

src/compiler/nir/nir_liveness.c

@@ -250,6 +250,15 @@ search_for_use_after_instr(nir_instr *start, nir_ssa_def *def)
          return true;
       node = node->next;
    }
+
+   /* If uses are considered to be in the block immediately preceding the if
+    * so we need to also check the following if condition, if any.
+    */
+   nir_if *following_if = nir_block_get_following_if(start->block);
+   if (following_if && following_if->condition.is_ssa &&
+       following_if->condition.ssa == def)
+      return true;
+
    return false;
 }