Python: Fix bad join in SSA

On `pritomrajkhowa/LoopBound`:

```
Definitions.ql-3:SsaCompute::SsaComputeImpl::AdjacentUsesImpl::adjacentUseUse#ff ................. 4m35s
```

specifically

```
(376s) Tuple counts for SsaCompute::SsaComputeImpl::AdjacentUsesImpl::adjacentUseUse#ff/2@be04e9kp after 4m58s:
388843     ~0%     {4} r1 = JOIN Essa::TPhiFunction#fff_2#join_rhs WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::definesAt#ffff ON FIRST 1 OUTPUT Rhs.1, Lhs.0, Rhs.2, Rhs.3
3629812090 ~1%     {7} r2 = JOIN r1 WITH SsaCompute::SsaComputeImpl::variableUse#ffff ON FIRST 1 OUTPUT Lhs.0, Rhs.2, Rhs.3, Lhs.2, Lhs.3, Lhs.1, Rhs.1 'use1'
0          ~0%     {2} r3 = JOIN r2 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::adjacentVarRefs#fffff ON FIRST 5 OUTPUT Lhs.5, Lhs.6 'use1'
0          ~0%     {2} r4 = JOIN r3 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::firstUse#ff ON FIRST 1 OUTPUT Lhs.1 'use1', Rhs.1 'use2'

897141     ~0%     {2} r5 = SsaCompute::SsaComputeImpl::AdjacentUsesImpl::adjacentUseUseSameVar#ff UNION r4
                    return r5
```

Clearly we do not want to join on the variable so soon. So we unbind it
and get

```
(78s) Tuple counts for SsaCompute::SsaComputeImpl::AdjacentUsesImpl::adjacentUseUse#ff/2@40e0e6uv after 434ms:
3377959 ~2%     {4} r1 = SCAN SsaCompute::SsaComputeImpl::variableUse#ffff OUTPUT In.0, In.2, In.3, In.1 'use1'
1026855 ~2%     {4} r2 = JOIN r1 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::adjacentVarRefs#fffff ON FIRST 3 OUTPUT Lhs.0, Rhs.3, Rhs.4, Lhs.3 'use1'
129484  ~0%     {2} r3 = JOIN r2 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::definesAt#ffff_1230#join_rhs ON FIRST 3 OUTPUT Rhs.3, Lhs.3 'use1'
0       ~0%     {2} r4 = JOIN r3 WITH Essa::TPhiFunction#fff_2#join_rhs ON FIRST 1 OUTPUT Lhs.0, Lhs.1 'use1'
0       ~0%     {2} r5 = JOIN r4 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::firstUse#ff ON FIRST 1 OUTPUT Lhs.1 'use1', Rhs.1 'use2'

897141  ~0%     {2} r6 = SsaCompute::SsaComputeImpl::AdjacentUsesImpl::adjacentUseUseSameVar#ff UNION r5
                return r6
```

Author: tausbn

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

@@ -478,12 +478,11 @@ private module SsaComputeImpl {
     predicate adjacentUseUse(ControlFlowNode use1, ControlFlowNode use2) {
       adjacentUseUseSameVar(use1, use2)
       or
-      exists(SsaSourceVariable v, EssaDefinition def, BasicBlock b1, int i1, BasicBlock b2, int i2 |
+      exists(SsaSourceVariable v, PhiFunction def, BasicBlock b1, int i1, BasicBlock b2, int i2 |
         adjacentVarRefs(v, b1, i1, b2, i2) and
-        variableUse(v, use1, b1, i1) and
-        definesAt(def, v, b2, i2) and
-        firstUse(def, use2) and
-        def instanceof PhiFunction
+        variableUse(pragma[only_bind_into](v), use1, b1, i1) and
+        definesAt(def, pragma[only_bind_into](v), b2, i2) and
+        firstUse(def, use2)
       )
     }