Add a BinaryOverflow instruction format.

This will eventualy be used for add-with-carry and add-with-overflow type
instructions. For now it only serves as a representative of instruction formats
that have multiple_results=True and boxed_storage=False at the same time.

src/libcretonne/write.rs

@@ -155,6 +155,7 @@ pub fn write_instruction(w: &mut Write, func: &Function, inst: Inst) -> Result {
         Binary { opcode, args, .. } => writeln!(w, "{} {}, {}", opcode, args[0], args[1]),
         BinaryImm { opcode, lhs, rhs, .. } => writeln!(w, "{} {}, {}", opcode, lhs, rhs),
         BinaryImmRev { opcode, lhs, rhs, .. } => writeln!(w, "{} {}, {}", opcode, lhs, rhs),
+        BinaryOverflow { opcode, args, .. } => writeln!(w, "{} {}, {}", opcode, args[0], args[1]),
         Jump { opcode, ref data, .. } => writeln!(w, "{} {}", opcode, data),
         Branch { opcode, ref data, .. } => writeln!(w, "{} {}", opcode, data),
         BranchTable { opcode, arg, table, .. } => writeln!(w, "{} {}, {}", opcode, arg, table),