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
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1587f7b9fc1a16df8b01b4f5f9e3c949325160b2

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(-)