Python: Fix bad join in firstUse

This was what it looked like (at the point when I killed the evaluation):

```
Tuple counts for SsaCompute::SsaComputeImpl::AdjacentUsesImpl::firstUse#c5fa2be7#ff/2@i1#be98bwif after 1m50s:
274000     ~7%     {4} r1 = SCAN SsaCompute::SsaComputeImpl::AdjacentUsesImpl::definesAt#c5fa2be7#ffff OUTPUT In.1, In.0 'def', In.2, In.3
2731768000 ~1%     {7} r2 = JOIN r1 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::variableSourceUse#c5fa2be7#ffff ON FIRST 1 OUTPUT Rhs.0, Lhs.2, Lhs.3, Rhs.2, Rhs.3, Rhs.1 'use', Lhs.1 'def'
178000     ~4%     {2} r3 = JOIN r2 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::adjacentVarRefs#c5fa2be7#fffff ON FIRST 5 OUTPUT Lhs.6 'def', Lhs.5 'use'
                    return r3
```

And this is what it looks like now:

```
Tuple counts for SsaCompute::SsaComputeImpl::AdjacentUsesImpl::firstUse#c5fa2be7#ff/2@i1#f9d6ewsi after 207ms:
931353  ~2%     {4} r1 = SCAN SsaCompute::SsaComputeImpl::AdjacentUsesImpl::variableSourceUse#c5fa2be7#ffff OUTPUT In.0, In.2, In.3, In.1 'use'
1050477 ~0%     {4} r2 = JOIN r1 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::adjacentVarRefs#c5fa2be7#fffff_03412#join_rhs ON FIRST 3 OUTPUT Lhs.0, Rhs.3, Rhs.4, Lhs.3 'use'
506626  ~0%     {2} r3 = JOIN r2 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::definesAt#c5fa2be7#ffff_1230#join_rhs ON FIRST 3 OUTPUT Rhs.3 'def', Lhs.3 'use'
                return r3
```

Author: tausbn

python/ql/lib/semmle/python/essa/SsaCompute.qll

@@ -496,8 +496,8 @@ private module SsaComputeImpl {
     predicate firstUse(EssaDefinition def, ControlFlowNode use) {
       exists(SsaSourceVariable v, BasicBlock b1, int i1, BasicBlock b2, int i2 |
         adjacentVarRefs(v, b1, i1, b2, i2) and
-        definesAt(def, v, b1, i1) and
-        variableSourceUse(v, use, b2, i2)
+        definesAt(def, pragma[only_bind_into](v), b1, i1) and
+        variableSourceUse(pragma[only_bind_into](v), use, b2, i2)
       )
       or
       exists(