Python: Fix bad join in MRO `flatten_list`
Python: Fix bad join in MRO `flatten_list`

This bad join was identified by the join-order-badness report, which
showed that:

py/use-of-input:MRO::flatten_list#f4eaf05f#fff#9c5fe54whnlqffdgu65vhb8uhpg# (order_500000)

calculated a whopping 212,820,108 tuples in order to produce an output of
size 55516, roughly 3833 times more effort than needed.

Here's a snippet of the slowest iteration of that predicate:
```
Tuple counts for MRO::flatten_list#f4eaf05f#fff/3@i1839#0265eb3w after 14ms:
0     ~0%     {3} r1 = JOIN MRO::need_flattening#f4eaf05f#f#prev_delta WITH MRO::ConsList#f4eaf05f#fff#reorder_2_0_1#prev ON FIRST 1 OUTPUT Rhs.1, Lhs.0 'list', Rhs.2
0     ~0%     {3} r2 = JOIN r1 WITH MRO::ClassList::length#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.2, Lhs.1 'list', Rhs.1 'n'
0     ~0%     {3} r3 = JOIN r2 WITH MRO::ClassListList::flatten#dispred#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1 'list', Lhs.2 'n', Rhs.1 'result'

0     ~0%     {3} r4 = SCAN MRO::ConsList#f4eaf05f#fff#prev_delta OUTPUT In.2 'list', In.0, In.1
0     ~0%     {3} r5 = JOIN r4 WITH MRO::need_flattening#f4eaf05f#f#prev ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Lhs.0 'list'
0     ~0%     {3} r6 = JOIN r5 WITH MRO::ClassList::length#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1, Lhs.2 'list', Rhs.1 'n'
0     ~0%     {3} r7 = JOIN r6 WITH MRO::ClassListList::flatten#dispred#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1 'list', Lhs.2 'n', Rhs.1 'result'

0     ~0%     {3} r8 = r3 UNION r7

26355 ~2%     {3} r9 = SCAN MRO::ConsList#f4eaf05f#fff#prev OUTPUT In.2 'list', In.0, In.1

0     ~0%     {3} r10 = JOIN r9 WITH MRO::need_flattening#f4eaf05f#f#prev ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Lhs.0 'list'
0     ~0%     {3} r11 = JOIN r10 WITH MRO::ClassList::length#f0820431#ff#prev_delta ON FIRST 1 OUTPUT Lhs.1, Lhs.2 'list', Rhs.1 'n'
0     ~0%     {3} r12 = JOIN r11 WITH MRO::ClassListList::flatten#dispred#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1 'list', Lhs.2 'n', Rhs.1 'result'
...
```
(... and a bunch more lines. The same construction appears several times,
but the join order is the same each time.)

Clearly it would be better to start with whatever is in `need_flattening`,
and then do the other joins. This is what the present fix does (by
unbinding `list` in all but the `needs_flattening` call).

After the fix, the slowest iteration is as follows:

```
Tuple counts for MRO::flatten_list#f4eaf05f#fff/3@i2617#8155ab3w after 9ms:
0 ~0%     {2} r1 = SCAN MRO::need_flattening#f4eaf05f#f#prev_delta OUTPUT In.0 'list', In.0 'list'

0 ~0%     {3} r2 = JOIN r1 WITH MRO::ConsList#f4eaf05f#fff#reorder_2_0_1#prev ON FIRST 1 OUTPUT Rhs.1, Lhs.1 'list', Rhs.2
0 ~0%     {3} r3 = JOIN r2 WITH MRO::ClassList::length#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.2, Lhs.1 'list', Rhs.1 'n'
0 ~0%     {3} r4 = JOIN r3 WITH MRO::ClassListList::flatten#dispred#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1 'list', Lhs.2 'n', Rhs.1 'result'

1 ~0%     {2} r5 = SCAN MRO::need_flattening#f4eaf05f#f#prev OUTPUT In.0 'list', In.0 'list'

0 ~0%     {3} r6 = JOIN r5 WITH MRO::ConsList#f4eaf05f#fff#reorder_2_0_1#prev_delta ON FIRST 1 OUTPUT Rhs.1, Lhs.1 'list', Rhs.2
0 ~0%     {3} r7 = JOIN r6 WITH MRO::ClassList::length#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.2, Lhs.1 'list', Rhs.1 'n'
0 ~0%     {3} r8 = JOIN r7 WITH MRO::ClassListList::flatten#dispred#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1 'list', Lhs.2 'n', Rhs.1 'result'
...
```
(... and so on. The remainder is 0 tuples all the way.)

In total, we went from
```
40.6s |  7614 |  15ms @ 1839 | MRO::flatten_list#f4eaf05f#fff@0265eb3w
```
to
```
7.8s |  7614 |  11ms @ 2617 | MRO::flatten_list#f4eaf05f#fff@8155ab3w
```

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