pgsql: Save a few cycles in plpgsql simple-expression initialization.

Save a few cycles in plpgsql simple-expression initialization.

Instead of using ExecPrepareExpr, call ExecInitExpr.  The net change here
is that we don't apply expression_planner() to the expression tree.  There
is no need to do so, because that tree is extracted from a fully planned
plancache entry, so all the needed work is already done.  This reduces
the setup costs by about a factor of 2 according to some simple tests.
Oversight noted while fooling around with the simple-expression code for
previous fix.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=37e0a016547abb3526b685d7aacbd0a4d57f6937

Modified Files
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src/pl/plpgsql/src/pl_exec.c |    5 +++--
1 files changed, 3 insertions(+), 2 deletions(-)