opt: add outbound FK checks for upsert

This is a re-implementation of Justin's change 3d1dd0f, except that we now
perform the insertion check for all new rows instead of just the inserted rows.
We do this by utilizing the same column that would be used for a RETURNING
clause, which in some cases is of the form
`CASE WHEN canary IS NULL col1 ELSE col2`.

Release note: None

pkg/sql/opt/optbuilder/mutation_builder.go

@@ -1013,15 +1013,22 @@ func (mb *mutationBuilder) buildFKChecksForUpdate() {
 }
 
 func (mb *mutationBuilder) buildFKChecksForUpsert() {
-	if mb.tab.OutboundForeignKeyCount() == 0 && mb.tab.InboundForeignKeyCount() == 0 {
+	numOutbound := mb.tab.OutboundForeignKeyCount()
+	numInbound := mb.tab.InboundForeignKeyCount()
+
+	if numOutbound == 0 && numInbound == 0 {
 		return
 	}
 	if !mb.b.evalCtx.SessionData.OptimizerFKs {
 		mb.fkFallback = true
 		return
 	}
-	// TODO(justin): not implemented yet.
-	mb.fkFallback = true
+
+	mb.withID = mb.b.factory.Memo().NextWithID()
+
+	for i := 0; i < numOutbound; i++ {
+		mb.addInsertionCheck(i)
+	}
 }
 
 // addInsertionCheck adds a FK check for rows which are added to a table.

pkg/sql/opt/optbuilder/testdata/fk-checks-insert

@@ -32,6 +32,44 @@ insert child
                 └── filters
                      └── column2:5 = parent.p:6
 
+build
+INSERT INTO child VALUES (100, 1), (200, 1) ON CONFLICT DO NOTHING
+----
+insert child
+ ├── columns: <none>
+ ├── insert-mapping:
+ │    ├── column1:3 => c:1
+ │    └── column2:4 => child.p:2
+ ├── input binding: &1
+ ├── project
+ │    ├── columns: column1:3!null column2:4!null
+ │    └── select
+ │         ├── columns: column1:3!null column2:4!null c:5 child.p:6
+ │         ├── left-join (hash)
+ │         │    ├── columns: column1:3!null column2:4!null c:5 child.p:6
+ │         │    ├── values
+ │         │    │    ├── columns: column1:3!null column2:4!null
+ │         │    │    ├── (100, 1)
+ │         │    │    └── (200, 1)
+ │         │    ├── scan child
+ │         │    │    └── columns: c:5!null child.p:6!null
+ │         │    └── filters
+ │         │         └── column1:3 = c:5
+ │         └── filters
+ │              └── c:5 IS NULL
+ └── f-k-checks
+      └── f-k-checks-item: child(p) -> parent(p)
+           └── anti-join (hash)
+                ├── columns: column2:7!null
+                ├── with-scan &1
+                │    ├── columns: column2:7!null
+                │    └── mapping:
+                │         └──  column2:4 => column2:7
+                ├── scan parent
+                │    └── columns: parent.p:8!null
+                └── filters
+                     └── column2:7 = parent.p:8
+
 # Use a non-constant input.
 exec-ddl
 CREATE TABLE xy (x INT, y INT)