Reduce ExecSeqScan* code size using pg_assume()

fb9f955025 optimized code generation by using specialized variants of
ExecSeqScan* for [not] having a qual, projection etc. This allowed the
compiler to optimize the code out the code for qual / projection. However, as
observed by David Rowley at the time, the compiler couldn't prove the
opposite, i.e. that the qual etc *are* present.

By using pg_assume(), introduced in d65eb5b1b8, we can tell the compiler that
the relevant variables are non-null.

This reduces the code size to a surprising degree and seems to lead to a small
but reproducible performance gain.

Reviewed-by: Amit Langote <amitlangote09@gmail.com> Discussion:
https://postgr.es/m/CA+HiwqFk-MbwhfX_kucxzL8zLmjEt9MMcHi2YF=DyhPrSjsBEA@mail.gmail.com

src/backend/executor/nodeSeqscan.c

@@ -131,8 +131,12 @@ ExecSeqScanWithQual(PlanState *pstate)
 {
 	SeqScanState *node = castNode(SeqScanState, pstate);
 
+	/*
+	 * Use pg_assume() for != NULL tests to make the compiler realize no
+	 * runtime check for the field is needed in ExecScanExtended().
+	 */
 	Assert(pstate->state->es_epq_active == NULL);
-	Assert(pstate->qual != NULL);
+	pg_assume(pstate->qual != NULL);
 	Assert(pstate->ps_ProjInfo == NULL);
 
 	return ExecScanExtended(&node->ss,
@@ -153,7 +157,7 @@ ExecSeqScanWithProject(PlanState *pstate)
 
 	Assert(pstate->state->es_epq_active == NULL);
 	Assert(pstate->qual == NULL);
-	Assert(pstate->ps_ProjInfo != NULL);
+	pg_assume(pstate->ps_ProjInfo != NULL);
 
 	return ExecScanExtended(&node->ss,
 							(ExecScanAccessMtd) SeqNext,
@@ -173,8 +177,8 @@ ExecSeqScanWithQualProject(PlanState *pstate)
 	SeqScanState *node = castNode(SeqScanState, pstate);
 
 	Assert(pstate->state->es_epq_active == NULL);
-	Assert(pstate->qual != NULL);
+	pg_assume(pstate->qual != NULL);
-	Assert(pstate->ps_ProjInfo != NULL);
+	pg_assume(pstate->ps_ProjInfo != NULL);
 
 	return ExecScanExtended(&node->ss,
 							(ExecScanAccessMtd) SeqNext,