Add the SQLITE_NOTNULL P5 code for comparison operations - really a composite

Add the SQLITE_NOTNULL P5 code for comparison operations - really a composite
of SQLITE_NULLEQ and SQLITE_JUMPIFNULL.  This flag indicates that NULL operands
are not possible and raises and assert() if NULL operands are seen.
Also omit an unnecessary scan of the sqlite_sequence table when writing
into an AUTOINCREMENT table.

FossilOrigin-Name: d2c047f304848e49864ed8c216b48fd671fa3916