Fix auto dem condition flow

[inmzhang]

In my understanding, the condition flow in the sinter.WorkIn.auto_dem() should be:

(decompose_errors=True, flatten_loops=False) -> (decompose_errors=True, flatten_loops=True) ->(decompose_errors=False, flatten_loops=False) ->(decompose_errors=False, flatten_loops=True)

while the current code tried (decompose_errors=True, flatten_loops=False) and (decompose_errors=False, flatten_loops=False) twice, which seems like typos. Did I miss something?

[Strilanc]

Urgh that's embarrassing that I got the order wrong. But I think this PR still has the wrong order. It should be

{flatten,decompose}
{decompose}
{flatten}
{}

The reason for flatten+decompose first is because flattening makes the conversion faster and the dem's file size smaller. There's a bug where it can cause the decomposition to fail, so there's the fallback to no-flattening. Probably this should just be changed to a fallback to no-flattening yes-decompose with ignore_decomposition_failures=True.

I'll think about it some more.

[Strilanc]

Moved this change into #832