[pseudo] Store last node popped in the queue, not its parent(s). NFC

We have to walk up to the last node to find the start token, but no need
to go even one node further.

This is one node fewer to store, but more importantly if the last node
happens to have multiple parents we avoid storing the sequence multiple times.

This saves ~5% on glrParse.
Based on a comment by hokein@ on https://reviews.llvm.org/D128307