Python: Fix bad join order for `py/multiple-calls-to-init`.
Python: Fix bad join order for `py/multiple-calls-to-init`.

The `multiple_invocation_paths` predicate had a bad join order where
we (essentially) joined `i1` with `i2` and only then joined `i1` and `i2`
separately to reduce the number of tuples. The join coming from `i1 != i2` had
little impact, but `i1.getFunction() = multi` made a big difference (and
similarly for `i2`). I factored out the code so that these joins would be done
more eagerly. Thus, we went from

```
[2019-11-06 16:53:05] (38s) Starting to evaluate predicate MethodCallOrder::multiple_invocation_paths#ffff/4@2ce75a
[2019-11-06 16:53:35] (68s) Tuple counts for MethodCallOrder::multiple_invocation_paths#ffff:
                      134547    ~9%       {2} r1 = SCAN CallGraph::TInvocation#fff AS I OUTPUT I.<0>, I.<2>
                      235284431 ~3%       {4} r2 = JOIN r1 WITH CallGraph::TInvocation#fff AS R ON FIRST 1 OUTPUT r1.<0>, r1.<1>, R.<1>, R.<2>
                      235149884 ~3%       {4} r3 = SELECT r2 ON r2.<3> != r2.<1>
                      235149884 ~4%       {3} r4 = SCAN r3 OUTPUT r3.<1>, r3.<0>, r3.<3>
                      166753634 ~5%       {4} r5 = JOIN r4 WITH #CallGraph::FunctionInvocation::getACallee_dispred#ffPlus#swapped AS R ON FIRST 1 OUTPUT R.<1>, r4.<2>, r4.<1>, r4.<0>
                      129778    ~0%       {4} r6 = JOIN r5 WITH #CallGraph::FunctionInvocation::getACallee_dispred#ffPlus AS R ON FIRST 2 OUTPUT r5.<0>, r5.<3>, r5.<1>, r5.<2>
                                          return r6
[2019-11-06 16:53:35] (68s) Registering MethodCallOrder::multiple_invocation_paths#ffff + [] with content 1705dcbc08kd9aa40rp2g2e9civhv
[2019-11-06 16:53:35] (68s)  >>> Wrote relation MethodCallOrder::multiple_invocation_paths#ffff with 129778 rows and 4 columns.
```

to

```
[2019-11-06 17:22:22] (25s) Starting to evaluate predicate MethodCallOrder::multiple_invocation_paths_helper#ffff/4@586aec
[2019-11-06 17:22:22] (25s) Tuple counts for MethodCallOrder::multiple_invocation_paths_helper#ffff:
                      134547 ~0%     {2} r1 = SCAN CallGraph::TInvocation#fff AS I OUTPUT I.<2>, I.<0>
                      88111  ~4%     {3} r2 = JOIN r1 WITH #CallGraph::FunctionInvocation::getACallee_dispred#ffPlus#swapped AS R ON FIRST 1 OUTPUT R.<1>, r1.<1>, r1.<0>
                      761305 ~0%     {4} r3 = JOIN r2 WITH #CallGraph::FunctionInvocation::getACallee_dispred#ffPlus AS R ON FIRST 1 OUTPUT r2.<1>, r2.<2>, r2.<0>, R.<1>
                      673194 ~0%     {4} r4 = SELECT r3 ON r3.<3> != r3.<1>
                      673194 ~0%     {4} r5 = SCAN r4 OUTPUT r4.<2>, r4.<1>, r4.<3>, r4.<0>
                                     return r5
[2019-11-06 17:22:22] (25s) Registering MethodCallOrder::multiple_invocation_paths_helper#ffff + [] with content 20edaaecf25nldgp24d9c4et8m3kv
[2019-11-06 17:22:22] (25s)  >>> Wrote relation MethodCallOrder::multiple_invocation_paths_helper#ffff with 673194 rows and 4 columns.
[2019-11-06 17:22:22] (25s) Starting to evaluate predicate MethodCallOrder::multiple_invocation_paths_helper#ffff_2301#join_rhs/4@9e5441
[2019-11-06 17:22:22] (25s) Tuple counts for MethodCallOrder::multiple_invocation_paths_helper#ffff_2301#join_rhs:
                      673194 ~0%     {4} r1 = SCAN MethodCallOrder::multiple_invocation_paths_helper#ffff AS I OUTPUT I.<2>, I.<3>, I.<0>, I.<1>
                                     return r1
[2019-11-06 17:22:22] (25s) Registering MethodCallOrder::multiple_invocation_paths_helper#ffff_2301#join_rhs + [] with content 2069301e655fi9mcovngg9hetfqas
[2019-11-06 17:22:22] (25s)  >>> Wrote relation MethodCallOrder::multiple_invocation_paths_helper#ffff_2301#join_rhs with 673194 rows and 4 columns.
[2019-11-06 17:22:22] (25s) Starting to evaluate predicate MethodCallOrder::multiple_invocation_paths#ffff/4@2f7c34
[2019-11-06 17:22:22] (25s) Tuple counts for MethodCallOrder::multiple_invocation_paths#ffff:
                      134547 ~0%     {2} r1 = SCAN CallGraph::TInvocation#fff AS I OUTPUT I.<2>, I.<0>
                      129778 ~0%     {4} r2 = JOIN r1 WITH MethodCallOrder::multiple_invocation_paths_helper#ffff_2301#join_rhs AS R ON FIRST 2 OUTPUT R.<2>, R.<3>, r1.<0>, r1.<1>
                                     return r2
[2019-11-06 17:22:22] (25s) Registering MethodCallOrder::multiple_invocation_paths#ffff + [] with content 1705dcbc08kd9aa40rp2g2e9civhv
[2019-11-06 17:22:22] (25s)  >>> Wrote relation MethodCallOrder::multiple_invocation_paths#ffff with 129778 rows and 4 columns.
[2019-11-06 17:22:22] (25s) Starting to evaluate predicate MethodCallOrder::multiple_invocation_paths#ffff_0312#join_rhs/4@9f9146
[2019-11-06 17:22:22] (25s) Tuple counts for MethodCallOrder::multiple_invocation_paths#ffff_0312#join_rhs:
                      129778 ~0%     {4} r1 = SCAN MethodCallOrder::multiple_invocation_paths#ffff AS I OUTPUT I.<0>, I.<3>, I.<1>, I.<2>
                                     return r1
[2019-11-06 17:22:22] (25s) Registering MethodCallOrder::multiple_invocation_paths#ffff_0312#join_rhs + [] with content 17c3fe1fcbf6ghhdr7hiukqp41rst
[2019-11-06 17:22:22] (25s)  >>> Wrote relation MethodCallOrder::multiple_invocation_paths#ffff_0312#join_rhs with 129778 rows and 4 columns.
```

Execution time on `salt` went from 29.5s to somewhere below 299ms (the predicate
was not listed in the timing report).

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