C#: Fix bad join in SSA library

```
[2023-08-29 10:10:29] Evaluated non-recursive predicate SsaImpl#75014cd4::Cached::lastRefBeforeRedefExt#4#ffff@4207c208 in 27604ms (size: 7511062).
Evaluated relational algebra for predicate SsaImpl#75014cd4::Cached::lastRefBeforeRedefExt#4#ffff@4207c208 with tuple counts:
           9905038   ~9%    {5} r1 = SCAN Ssa#da392372::Make#SsaImpl#75014cd4::SsaInput#::lastRefRedefExt#5#fffff OUTPUT In.2, In.3, In.1, In.0, In.4
                            {5} r2 = r1 AND NOT _SsaImpl#75014cd4::SsaInput::variableRead#4#ffff_3012#join_rhs_const_false#antijoin_rhs(Lhs.0, Lhs.1, Lhs.2)
           4605608   ~0%    {4} r3 = SCAN r2 OUTPUT In.3, In.0, In.1, In.4

        4510888816   ~0%    {5} r4 = JOIN _SsaImpl#75014cd4::SsaInput::variableRead#4#ffff_3012#join_rhs_const_false#antijoin_rhs WITH project#Ssa#da392372::Make#SsaImpl#75014cd4::SsaInput#::lastRefRedefExt#5#fffff_1203#join_rhs ON FIRST 2 OUTPUT Rhs.2, Lhs.2, Lhs.0, Lhs.1, Rhs.3
           5294405  ~82%    {4} r5 = JOIN r4 WITH SsaImpl#75014cd4::adjacentDefReachesReadExt#6#ffffff_014523#join_rhs ON FIRST 4 OUTPUT Lhs.0, Rhs.4, Rhs.5, Lhs.4

           9900013  ~28%    {4} r6 = r3 UNION r5
                            return r6
```

Author: hvitved

csharp/ql/lib/semmle/code/csharp/dataflow/internal/SsaImpl.qll

@@ -1159,6 +1159,7 @@ private predicate adjacentDefReachesUncertainRead(
   )
 }
 
+pragma[nomagic]
 private predicate adjacentDefReachesUncertainReadExt(
   DefinitionExt def, SsaInput::BasicBlock bb1, int i1, SsaInput::BasicBlock bb2, int i2
 ) {