Add trap codes to the Cretonne IL.

The trap and trapz/trapnz instructions now take a trap code immediate
operand which indicates the reason for trapping.

lib/cretonne/src/write.rs

@@ -376,6 +376,8 @@ pub fn write_operands(
                 write!(w, " {}, %{} -> %{}", arg, src, dst)
             }
         }
+        Trap { code, .. } => write!(w, " {}", code),
+        CondTrap { arg, code, .. } => write!(w, " {}, {}", arg, code),
     }
 }