Delay commit status checks until freezing executes.

pg_xact lookups are relatively expensive.  Move the xmin/xmax commit
status checks from the point that freeze plans are prepared to the point
that they're actually executed.  Otherwise we'll repeat many commit
status checks whenever multiple successive VACUUM operations scan the
same pages and decide against freezing each time, which is a waste of
cycles.

Oversight in commit 1de58df4, which added page-level freezing.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzkZpe4K6qMfEt8H4qYJCKc2R7TPvKsBva7jc9w7iGXQSw@mail.gmail.com

src/include/access/heapam.h

@@ -100,6 +100,13 @@ typedef enum
 	HEAPTUPLE_DELETE_IN_PROGRESS	/* deleting xact is still in progress */
 } HTSV_Result;
 
+/*
+ * heap_prepare_freeze_tuple may request that heap_freeze_execute_prepared
+ * check any tuple's to-be-frozen xmin and/or xmax status using pg_xact
+ */
+#define		HEAP_FREEZE_CHECK_XMIN_COMMITTED	0x01
+#define		HEAP_FREEZE_CHECK_XMAX_ABORTED		0x02
+
 /* heap_prepare_freeze_tuple state describing how to freeze a tuple */
 typedef struct HeapTupleFreeze
 {
@@ -109,6 +116,8 @@ typedef struct HeapTupleFreeze
 	uint16		t_infomask;
 	uint8		frzflags;
 
+	/* xmin/xmax check flags */
+	uint8		checkflags;
 	/* Page offset number for tuple */
 	OffsetNumber offset;
 } HeapTupleFreeze;