pgsql: Evade extra table_tuple_fetch_row_version() in ExecUpdate()/Exec

Evade extra table_tuple_fetch_row_version() in ExecUpdate()/ExecDelete()

When we lock tuple using table_tuple_lock() then we at the same time fetch
the locked tuple to the slot. In this case we can skip extra
table_tuple_fetch_row_version() thank to we've already fetched the 'old' tuple
and nobody can change it concurrently since it's locked.

Discussion: https://postgr.es/m/CAPpHfdua-YFw3XTprfutzGp28xXLigFtzNbuFY8yPhqeq6X5kg%40mail.gmail.com
Reviewed-by: Aleksander Alekseev, Pavel Borisov, Vignesh C, Mason Sharp
Reviewed-by: Andres Freund, Chris Travers

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/764da7710bf68eebb2c0facb2f871bc3c7a705b6

Modified Files
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src/backend/executor/nodeModifyTable.c | 48 +++++++++++++++++++++++++---------
1 file changed, 35 insertions(+), 13 deletions(-)