stage2: implement TZIR printing for `block` and `condbr`

Zig code:
```
fn assert(ok: bool) void {
    if (!ok) unreachable;
}
```

TZIR:
```
Module.Function(name=assert):
  @0: void = {};
  %0: bool = arg(ok)
  %1: void = dbg_stmt()
  %2: void = block(
    %3: bool = not(%0)
    %4: noreturn = condbr(%3,
    then:
      %5: void = breakpoint()
      %6: noreturn = unreach()
    else:
      %7: noreturn = br(%2, @0)
    )
  )
  %8: noreturn = retvoid()
```

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