Attempt to simplify the logic and generated code for vector comparisons.

Basic comparison operators are working, but there are many indexing test
failures still to be worked through.

FossilOrigin-Name: dfc028cfbe7657d20727a2670ecadb1575eb8cbb

src/sqliteInt.h

@@ -1733,6 +1733,7 @@ struct CollSeq {
 ** operator is NULL.  It is added to certain comparison operators to
 ** prove that the operands are always NOT NULL.
 */
+#define SQLITE_KEEPNULL     0x08  /* Used by vector == or <> */
 #define SQLITE_JUMPIFNULL   0x10  /* jumps if either operand is NULL */
 #define SQLITE_STOREP2      0x20  /* Store result in reg[P2] rather than jump */
 #define SQLITE_NULLEQ       0x80  /* NULL=NULL */