z port: use unsigned comparison for Icheckbound instructions
z port: use unsigned comparison for Icheckbound instructions

This avoids testing the "< 0" case separately.

eliberis/ocaml,gerdstolpmann/ocaml,yunxing/ocaml,gerdstolpmann/ocaml,eliberis/ocaml,msprotz/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,eliberis/ocaml,yunxing/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,yunxing/ocaml,gerdstolpmann/ocaml,eliberis/ocaml,yunxing/ocaml,msprotz/ocaml,msprotz/ocaml,eliberis/ocaml,yunxing/ocaml