sql/randgen: avoid invalid inverted indexes in CREATE TABLE

Previously, the create table made via the randgen component
could create invalid CREATE TABLE statements where the last
columns of an inverted index were descending. This could
lead to unexpected failures in certain workloads, which
expected valid statements from this components. To address
this, this patch stops randgen from generating CREATE table
statements with invalid inverted indexes.

Release note: None

pkg/sql/randgen/schema.go

@@ -189,6 +189,10 @@ func RandCreateTableWithColumnIndexNumberGenerator(
 			if foundOverlap {
 				continue
 			}
+			// Inverted indexes can only be ascending on the last column.
+			if col.Direction == tree.Descending {
+				continue
+			}
 		}
 		// Make forward indexes unique 50% of the time. Inverted indexes cannot
 		// be unique.