Python: Fix several bad `getScope` joins
Python: Fix several bad `getScope` joins

It seems the optimiser has started getting the wrong end of the stick
whenever we write `foo.getScope() = bar.getScope()` for some expressions
`foo` and `bar`.

This lead to things like

```
(196s) Tuple counts for Definitions::ModuleVariable::global_variable_callnode#ff/2@5ab278 after 2m33s:
2952757013 ~0%     {2} r1 = JOIN Definitions::ModuleVariable::global_variable_callnode#ff#shared WITH Variables::Variable::getScope_dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1 'this', Lhs.1 'result'
495693     ~0%     {2} r2 = JOIN r1 WITH Variables::GlobalVariable#class#f ON FIRST 1 OUTPUT Lhs.0 'this', Lhs.1 'result'
453589     ~0%     {2} r3 = JOIN r2 WITH Definitions::ModuleVariable#f ON FIRST 1 OUTPUT Lhs.0 'this', Lhs.1 'result'
                   return r3
```

and

```
(315s) Tuple counts for Definitions::SsaSourceVariable::getAUse_dispred#ff/2@a39328 after 1m57s:
...
1785275    ~3%       {2} r24 = Definitions::ModuleVariable::global_variable_callnode#ff#shared UNION Definitions::SsaSourceVariable::getAUse_dispred#ff#shared
3008614987 ~0%       {2} r25 = JOIN r24 WITH Variables::Variable::getScope_dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1 'this', Lhs.1 'result'
127        ~1%       {2} r26 = JOIN r25 WITH Definitions::NonLocalVariable#class#f ON FIRST 1 OUTPUT Lhs.0 'this', Lhs.1 'result'
127        ~1%       {2} r27 = JOIN r26 WITH Variables::LocalVariable#f ON FIRST 1 OUTPUT Lhs.0 'this', Lhs.1 'result'
...
```

(Note the timings: 2m33s and 1m57s.)

Now we have the much more reasonable

```
(38s) Tuple counts for Definitions::ModuleVariable::global_variable_callnode#ff/2@c53031 after 42ms:
453589 ~0%     {2} r1 = JOIN Definitions::ModuleVariable::global_variable_callnode#ff#shared WITH Definitions::ModuleVariable::scope_as_global_variable#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1 'this', Lhs.1 'result'
               return r1
```

and

```
(46s) Tuple counts for Definitions::SsaSourceVariable::getAUse_dispred#ff/2@4b19de after 375ms:
...
```

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