htools: replace two hardcoded uses of pri+sec nodes

These two cases use explicit uses of primary and secondary nodes with
Instance.allNodes, which means the code is more flexible if the
internal layout of the instance changes.

I've verified that the output of involvedNodes  is not required to be
4-element long, and as such the function docstring has been updated.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>