pgsql: Merge copies of converting an XID to a FullTransactionId.

Merge copies of converting an XID to a FullTransactionId.

Assume twophase.c is the performance-sensitive caller, and preserve its
choice of unlikely() branch hint.  Add some retrospective rationale for
that choice.  Back-patch to v17, for the next commit to use it.

Reviewed (in earlier versions) by Michael Paquier.

Discussion: https://postgr.es/m/17821-dd8c334263399284@postgresql.org
Discussion: https://postgr.es/m/20250116010051.f3.nmisch@google.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/81772a495ec98d36eabf5cc294e7031a9545c5c1

Modified Files
--------------
contrib/amcheck/verify_heapam.c         |  4 +-
src/backend/access/transam/twophase.c   | 26 +++--------
src/backend/access/transam/xlogreader.c | 18 +-------
src/backend/utils/adt/xid8funcs.c       | 79 ++++++++++-----------------------
src/include/access/transam.h            | 43 ++++++++++++++++++
5 files changed, 77 insertions(+), 93 deletions(-)