Unnecessary line break before list comprehension

Nobody has claimed this yet.

```haskell
foo =
let r =
[ m | m <- foooooooooooooooooooooooooooooooooooooooooooooooooooooooo ]
in ()
```

The break between `r =` and `[ m` is a little odd here.

This happens also without a single very long word

```haskell
foo =
let r =
[ m
| m <- foooooooooooooooooooo
, fooooooooooooooooooooooooooo
, fooooooooooooooooooooooooooo
]
in ()
```

Start by reproducing both Haskell snippets with Brittany and compare the formatter's output with the examples in this issue. Trace the formatting path that decides the break after `r =`, then verify that the list-comprehension layout is corrected for both the long-word and multi-clause cases without changing unrelated formatting.

Written by the indexing model from the issue text.