Handle Duff's device and other unstructured switch statements

[xavierleroy]

This PR introduces a new (unverified) elaboration pass, SwitchNorm, that recognizes structured switch statements and puts them in a normalized form. If selected (language support option -funstructured-switch), other switch statements are turned into a structured switch with goto actions, followed by the original switch body with appropriate labels and goto.

For example, Duff's infamous code

  switch (count % 8) {
    case 0: do { *to++ = *from++;
    case 7:      *to++ = *from++;
    case 6:      *to++ = *from++;
    case 5:      *to++ = *from++;
    case 4:      *to++ = *from++;
    case 3:      *to++ = *from++;
    case 2:      *to++ = *from++;
    case 1:      *to++ = *from++;
            } while (--n > 0);
  }

becomes

  switch (count % 8) {
    default:   goto @9;
    case 0:    goto @1;
    case 7:    goto @2;
    case 6:    goto @3;
    case 5:    goto @4;
    case 4:    goto @5;
    case 3:    goto @6;
    case 2:    goto @7;
    case 1:    goto @8;
  }
  @1:
  do {
    *to++ = *from++;
    @2:    *to++ = *from++;
    @3:    *to++ = *from++;
    @4:    *to++ = *from++;
    @5:    *to++ = *from++;
    @6:    *to++ = *from++;
    @7:    *to++ = *from++;
    @8:    *to++ = *from++;
  } while(--n > 0);
  @9:  /*skip*/;

Other examples can be found in test/regression/switch3.c.