pgsql: Avoid repeating loads of frozen ID values.

Avoid repeating loads of frozen ID values.

Repeating loads of inplace-updated fields tends to cause bugs like the
one from the previous commit.  While there's no bug to fix in these code
sites, adopt the load-once style.  This improves the chance of future
copy/paste finding the safe style.

Discussion: https://postgr.es/m/20240423003956.e7.nmisch@google.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/dd0183469bb779247c96e86c2272dca7ff4ec9e7

Modified Files
--------------
src/backend/access/heap/heapam.c    | 16 ++++++++++------
src/backend/commands/cluster.c      | 22 ++++++++++++++--------
src/backend/commands/vacuum.c       |  4 ++--
src/backend/postmaster/autovacuum.c | 13 ++++++++-----
4 files changed, 34 insertions(+), 21 deletions(-)