pgsql: Refactor code for cross-partition updates to a separate function

Refactor code for cross-partition updates to a separate function.

ExecUpdate() is very long, so extract the part of it that deals with
cross-partition updates to a separate function to make it more readable.
Per Andres Freund's suggestion.

Author: Amit Langote
Discussion:
https://www.postgresql.org/message-id/CA%2BHiwqEUgb5RdUgxR7Sqco4S09jzJstHiaT2vnCRPGR4JCAPqA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c5b097f8fa69861ab83470c0bfe732ab90df9e62

Modified Files
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src/backend/executor/nodeModifyTable.c | 265 ++++++++++++++++++++-------------
1 file changed, 158 insertions(+), 107 deletions(-)