private static boolean isConstIntZeroOrKnownNull(RegisterSpec spec) {
    TypeBearer tb = spec.getTypeBearer();
    if (tb instanceof CstLiteralBits) {
      CstLiteralBits clb = (CstLiteralBits) tb;
      return (clb.getLongBits() == 0);
    }
    return false;
  }
Returns true if the register contains an integer 0 or a known-null object reference

@param spec non-null spec
@return true for 0 or null type bearers
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/LiteralOpUpgrader.java