pgsql: Modernize some code a bit

Modernize some code a bit

Modernize code in ExecRelCheck() and ExecConstraints() a bit,
preparing the way for some new code.

Co-authored-by: jian he <jian.universality@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Navneet Kumar <thanit3111@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: /messages/by-id/CACJufxHArQysbDkWFmvK+D1TPHQWWTxWN15cMuUaTYX3xhQXgg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/747ddd38cbf6d32bca496e69c1efb2ae4fe333cc

Modified Files
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src/backend/executor/execMain.c | 35 ++++++++++++++++++-----------------
1 file changed, 18 insertions(+), 17 deletions(-)