At the end of what ? It does not help to set it at the end of CREATE INDEX because the transaction may not commit immediately. In the meantime, many new transactions may start with transaction id > xcreate. All these transactions can see the old tuple (which we did not index) and can also see the index once CREATE INDEX commits.
If you are indexing a table that hasn't just been created by you, set the xcreate field on pg_index at the *end* of the build using ReadNewTransactionId(). Any xid less than that sees the index as invalid. If you created the table in this transaction ( i.e. createSubId != 0) then set xcreate to creating xid.
Why do we need to handle the case where table is created in the same transaction ? Neither the table nor the index is visible until we commit. So thats a simple case.