Clarify a comment.

;; This vop is intended to handle the case (logand x #xffffffff) when

;; x is a (signed-byte 32).  We can just do a register move instead of

;; and'ing with #xffffffff.

;; and'ing with #xffffffff.  Some other special cases are handled too

;; when the constant can fit inside the immediate operaand of the and

;; instruction.

(define-vop (fast-logand-c/signed-unsigned=>unsigned fast-unsigned-binop-c)

  (:args (x :scs (signed-reg)))

  (:translate logand)

	  ((typep y '(unsigned-byte 13))

	   (inst and r x y))

	  (t

	   ;; The constant is really a (signed-byte 13), so convert it

	   ;; The constant operand is really a (signed-byte 13), so convert it

	   ;; to a negative number so the immediate operand gets

	   ;; signed extended to the right bits.

	   (inst and r x (- y #x100000000))