Idempotence issue with comments

[judah]

This is another case where Ormolu's handling of comments is non-idempotent. I'm filing it as a separate issue from #340 because it doesn't seem related to matches, and also seems more sensitive to the exact input than usual.

Original input:

x  =
      y ++ -- commentA
-- commentB
      f g -- commentC

The first run of Ormolu produces:

x =
  y
    ++ f g -- commentA
    -- commentB
      -- commentC

The second run produces:

x =
  y
    ++ f g -- commentA
      -- commentB
      -- commentC

[judah]

Also, on second look, the moving of commentB above f g also seems suspect.

As another example, if I replace f g with f then the result is idempotent:

x =
  y
    ++ f -- commentA
    -- commentB
    -- commentC

and the steady-state picks a different indentation for commentB and commentC than in the original example.