simplify: rewrite bitxnor on booleans to equal

This adds the rewrite (a xnor b) --> (a = b) for booleans a, b to the
simplifier.

Author: kroening

src/util/simplify_expr_int.cpp

@@ -696,6 +696,8 @@ simplify_exprt::simplify_bitwise(const multi_ary_exprt &expr)
         new_expr.id(ID_or);
       else if(expr.id()==ID_bitxor)
         new_expr.id(ID_xor);
+      else if(expr.id() == ID_bitxnor)
+        new_expr.id(ID_equal);
       else
         UNREACHABLE;