kconfig/menu.c: fix multiple references to expressions in menu_add_prop()
menu_add_prop() applies upper menus' visibilities to actual prompts
by AND-ing the prompts visibilities with the upper menus ones.
This creates a further reference to the menu's visibilities and when
the expression reduction functions do their work, they may remove or
modify expressions that have multiple references, thus causing
unpredictable side-effects.
The following example Kconfig constructs a case where this causes
problems: a menu and a prompt which's visibilities depend on the same
symbol. When invoking mconf with this Kconfig and pressing "Z" we
see a problem caused by a free'd expression still referenced by the
menu's visibility: