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.

src/pl/plpgsql/src/pl_exec.c

@@ -4577,10 +4577,11 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
 	 */
 	if (expr->expr_simple_lxid != curlxid)
 	{
-		expr->expr_simple_state = ExecPrepareExpr(expr->expr_simple_expr,
-												  simple_eval_estate);
+		oldcontext = MemoryContextSwitchTo(simple_eval_estate->es_query_cxt);
+		expr->expr_simple_state = ExecInitExpr(expr->expr_simple_expr, NULL);
 		expr->expr_simple_in_use = false;
 		expr->expr_simple_lxid = curlxid;
+		MemoryContextSwitchTo(oldcontext);
 	}
 
 	/*