Don't include UNUSED in list of all instructions

This was causing UNUSED instructions to be sampled more often than
expected

gbso/cpu/insn.py

@@ -1694,7 +1694,7 @@ class UNUSED(Insn):
 
 
 ALL_INSN_CLASSES: List[Type[Insn]] = [
-    cls for cls in Insn.__subclasses__() if cls != Insn  # type: ignore
+    cls for cls in Insn.__subclasses__() if cls not in {Insn, UNUSED}  # type: ignore
 ]
 
 INSNS_BY_SIGNATURE: Dict[str, List[Type[Insn]]] = defaultdict(list)