Fix null cmp codegen

[sivukhin]

This PR remove manual null-comparison optimization (introduced in #847) which replace Eq/Ne instructions with explicit IsNull/NotNull. There are few factors for this change:

1. Before, manual optimization were incorrect because it ignored jump_if_condition_is_true flag which is important to properly build logical condition evaluation
2. Manual optimization covered all scenarios in test cases and scenarios when both sides are non trivial expressions were not covered by tests
3. Limbo already mark literals in the initial emitted bytecode as constants and will evaluate and store them only once - so performance difference from manual optimization seems very minor to me (but I am wrong with high probability)
4. I think (but again, I am wrong with high probability) that such replacement can be done in the generic optimizator layer instead of manually encode them in the first emit phase

Fixes #850

[jussisaurio]

possible to add a test case that would capture 1. current main failing it 2. this branch succeeding?

diff --git a/testing/where.test b/testing/where.test
index d34cc1b..3259cc5 100755
--- a/testing/where.test
+++ b/testing/where.test
@@ -35,6 +35,11 @@ do_execsql_test_small where-not-equals-a-with-nulls {
     select count(*) from demo where value != 'A';
 } {2}
 
+do_execsql_test_small where-is-null-or-id-2 {
+    select id from demo where value is null or id == 2;
+} {2
+4}
+
 do_execsql_test where-clause-eq {
     select last_name from users where id = 2000;
 } {Rodriguez}

i verified this fails on main and succeeds here

EDIT: hmm I guess you added some in #857 ?