Use correct symbol for minimum int64 value

The old code used SEQ_MINVALUE to get the smallest int64 value.  This
was done as a convenience to avoid having to deal with INT64_IS_BUSTED,
but that is obsolete now.  Also, it is incorrect because the smallest
int64 value is actually SEQ_MINVALUE-1.  Fix by using PG_INT64_MIN.

contrib/btree_gin/btree_gin.c

@@ -223,10 +223,7 @@ GIN_SUPPORT(int4, false, leftmostvalue_int4, btint4cmp)
 static Datum
 leftmostvalue_int8(void)
 {
-	/*
-	 * Use sequence's definition to keep compatibility.
-	 */
-	return Int64GetDatum(SEQ_MINVALUE);
+	return Int64GetDatum(PG_INT64_MIN);
 }
 
 GIN_SUPPORT(int8, false, leftmostvalue_int8, btint8cmp)
@@ -250,10 +247,7 @@ GIN_SUPPORT(float8, false, leftmostvalue_float8, btfloat8cmp)
 static Datum
 leftmostvalue_money(void)
 {
-	/*
-	 * Use sequence's definition to keep compatibility.
-	 */
-	return Int64GetDatum(SEQ_MINVALUE);
+	return Int64GetDatum(PG_INT64_MIN);
 }
 
 GIN_SUPPORT(money, false, leftmostvalue_money, cash_cmp)