Incorrect value computed, conditional with = of integers

[Paul F. Dietz]

(defun bug236 (a)
  (declare (type number a))
  (declare (optimize debug))
  (setf a 1)
  (let ((v9 1))
    (let ((v1 (/ a 1)))
      (if (= v9 v1)
          :good
          :bad))))

(bug236 0) ==> :bad

x86-64, "2.3.8.151-8c8c49815"