[range-ops] Remove special case for handling bitmasks in casts.

Now that we can generically handle bitmasks for unary operators,
there's no need to special case them.

gcc/ChangeLog:

	* range-op-mixed.h (class operator_cast): Add update_bitmask.
	* range-op.cc (operator_cast::update_bitmask): New.
	(operator_cast::fold_range): Call update_bitmask.