private static RegisterSpec filterSpec(RegisterSpec orig) {
      if ((orig != null) && (orig.getType() == Type.KNOWN_NULL)) {
        return orig.withType(Type.OBJECT);
      }

      return orig;
    }
Converts a given spec into the form acceptable for use in a local list. This, in particular,
transforms the "known null" type into simply {@code Object}. This method needs to be called
for any spec that is on its way into a locals list.

<p>This isn't necessarily the cleanest way to achieve the goal of not representing known
nulls in a locals list, but it gets the job done.

@param orig {@code null-ok;} the original spec
@return {@code null-ok;} an appropriately modified spec, or the original if nothing needs to
    be done
android/android_framework/dex-maker/src/main/java/com/android/dx/dex/code/LocalList.java