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