Reorder two functions in inval.c

Reorder two functions in inval.c

This file separates public and static functions with a separator
comment, but two routines were not defined in a location reflecting
that, so reorder them.

Back-patch commit c2bdd2c5b1d48a7e39e1a8d5e1d90b731b53c4c9 to v15 - v16.
This avoids merge conflicts in the next commit, which modifies a
function this moved.  Exclude v14, which is so different that the merge
conflict savings would be immaterial.

Author: Aleksander Alekseev <aleksander@timescale.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAJ7c6TMX2dd0g91UKvcC+CVygKQYJkKJq1+ZzT4rOK42+b53=w@mail.gmail.com
Backpatch-through: 15-16