pgsql: Refactor checks for deleted GiST pages.

Refactor checks for deleted GiST pages.

The explicit check in gistScanPage() isn't currently really necessary, as
a deleted page is always empty, so the loop would fall through without
doing anything, anyway. But it's a marginal optimization, and it gives a
nice place to attach a comment to explain how it works.

Backpatch to v12, where GiST page deletion was introduced.

Reviewed-by: Andrey Borodin
Discussion: https://www.postgresql.org/message-id/835A15A5-F1B4-4446-A711-BF48357EB602%40yandex-team.ru

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e2e992c93145cfc0e3563fb84efd25b390a84bb9

Modified Files
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src/backend/access/gist/gist.c    | 40 +++++++++++++++------------------------
src/backend/access/gist/gistget.c | 14 ++++++++++++++
2 files changed, 29 insertions(+), 25 deletions(-)