Emission of unnecessary jumps

[xclerc]

The following code:

let[@inline never] failure _ = failwith "..."

type t =
  | A
  | B
  | C

let of_int_exn = function
  | 0 -> A
  | 1 -> B
  | 2 -> C
  | n -> failure n

generates:

        cmpq    $5, %rax
        jbe     .L1
        jmp     camlA__failure_0_2_code@PLT
.L1:
        ret

which is rather good, but contains one unnecessary jump.

The reason why it is not optimized away is that the CFG
representation does not see two jumps, but only a conditional
jump and a (direct) tail call.

It seems possible to slightly alter the CFG representation
so that for instance the destinations of conditional jumps
are not mere labels but symbols.

However, doing so would have an impact on a number
of existing passes, for instance the jump-on-jump optimization
or the zero-alloc checker.