Cleanup pointer_plus_operator.

The POINTER_PLUS operator still carries some remnamts of the old
irange interface, which is now dead code with prange.

	* range-op-ptr.cc (pointer_plus_operator::wi_fold): Remove.
	(pointer_plus_operator::op2_range): Remove irange variant.
	(pointer_plus_operator::update_bitmask): Likewise.