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.