Properly implement round-trip property for `if`/`else`

[sgraf812]

In #6188, @phadej is hesitating to delete dead code because

That dead code reminds me of an unsolved issue, but when it's gone there won't be anything reminding of that ugliness...

So I'm recording his process in #6188 (comment) here so that this issue can serve as a memory hook for him to come back and fix the code later

---

The code there is in the state to have roundtrip property for code like

if flag(some-condition)
  buildable: True
else
  buildable: False

which is ugly when pretty-printed as:

if flag(some-condition)
else
  buildable: False

That's because buildable: True is no-op, and there could be other a like.

One way, is to pretty-print that as

if !flag(some-condition)
  buildable: False

But that violates roundtrip property.

[phadej]

Slight duplicate of #6189