Evaluate arguments of correlated SubPlans in the referencing ExprState

Until now we generated an ExprState for each parameter to a SubPlan and
evaluated them one-by-one ExecScanSubPlan. That's sub-optimal as creating lots
of small ExprStates
a) makes JIT compilation more expensive
b) wastes memory
c) is a bit slower to execute

This commit arranges to evaluate parameters to a SubPlan as part of the
ExprState referencing a SubPlan, using the new EEOP_PARAM_SET expression
step. We emit one EEOP_PARAM_SET for each argument to a subplan, just before
the EEOP_SUBPLAN step.

It likely is worth using EEOP_PARAM_SET in other places as well, e.g. for
SubPlan outputs, nestloop parameters and - more ambitiously - to get rid of
ExprContext->domainValue/caseValue/ecxt_agg*.  But that's for later.

Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Alena Rybakina <lena.ribackina@yandex.ru>
Discussion: https://postgr.es/m/20230225214401.346ancgjqc3zmvek@awork3.anarazel.de

src/backend/executor/execProcnode.c

@@ -393,6 +393,10 @@ ExecInitNode(Plan *node, EState *estate, int eflags)
 	/*
 	 * Initialize any initPlans present in this node.  The planner put them in
 	 * a separate list for us.
+	 *
+	 * The defining characteristic of initplans is that they don't have
+	 * arguments, so we don't need to evaluate them (in contrast to
+	 * ExecInitSubPlanExpr()).
 	 */
 	subps = NIL;
 	foreach(l, node->initPlan)
@@ -401,6 +405,7 @@ ExecInitNode(Plan *node, EState *estate, int eflags)
 		SubPlanState *sstate;
 
 		Assert(IsA(subplan, SubPlan));
+		Assert(subplan->args == NIL);
 		sstate = ExecInitSubPlan(subplan, result);
 		subps = lappend(subps, sstate);
 	}