pgsql: Reduce code duplication for ExecJust*Var operations.

Reduce code duplication for ExecJust*Var operations.

This is mainly in preparation for adding further fastpath evaluation
routines.

Also reorder ExecJust*Var functions to be consistent with the order in
which they're used.

Author: Andres Freund
Discussion: https://postgr.es/m/CAE-ML+9OKSN71+mHtfMD-L24oDp8dGTfaVjDU6U+j+FNAW5kRQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/34c9c53bb035ba92491006eb80f7a60527db367d

Modified Files
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src/backend/executor/execExprInterp.c | 94 +++++++++++++----------------------
1 file changed, 35 insertions(+), 59 deletions(-)