Annotations by @mflatt.
0: ff0a00f1 (subs xzr x23 #x2) ; are there two arguments?
4: 01040054 (b.ne (+ pc #x80)) ; => 84 ; jump to slow path if not
8: 1e0001aa (orr x30 x0 x1) ; "or" to combine mask bits
c: df0b40f2 (ands xzr x30 #x7) ; zero low bits => both are fixnums
10: a1000054 (b.ne (+ pc #x14)) ; => 24 ; jump to slow path if not