Check if constant is a member before returning it.

Check if constant is a member before returning it.

set_range_from_bitmask checks the new bitmask, and if it is a constant,
simply returns the constant.  It never checks if that constant is
actually within the range.  If it is not, the result should be UNDEFINED.

	* value-range.cc (irange::set_range_from_bitmask): When the bitmask
	result is a singleton, check if it is contained in the range.