Add a trapif instruction.

This is a conditional trap controlled by integer CPU flags.
Compare to brif.

lib/cretonne/src/write.rs

@@ -428,6 +428,7 @@ pub fn write_operands(
         }
         Trap { code, .. } => write!(w, " {}", code),
         CondTrap { arg, code, .. } => write!(w, " {}, {}", arg, code),
+        IntCondTrap { cond, arg, code, .. } => write!(w, " {} {}, {}", cond, arg, code),
     }
 }