For methods compiled without optimization (and possibly also with optimization),
it is possible for a variable update to have multiple possible assigned values.
For example, the non-optimized CIL for
```
return cond ? null : "not null"
```
is
```
0: nop
1: ldarg.0
2: ldfld cond
3: brtrue.s 6:
4: ldstr "not null"
5: br.s 7:
6: ldnull
7: stloc.0 L0 // stores either `null` or "not null"
8: br.s 9:
9: ldloc.0
10: ret
```
Consequently, an existential in `CallableReturns.qll` must be a `forex`.
- Cache predicates in the same stage using a cached module.
- Introduce `DefUse::defUseVariableUpdate()` and use in `CallableReturns.qll`.
The updated file `csharp/ql/test/library-tests/cil/dataflow/Nullness.expected`
demonstrates why this is needed.
- Utilize CIL analysis in `Guards::nonNullValue()`.
- Analyze SSA definitions in `AlwaysNullExpr`, similar to `NonNullExpr`.
