Fix value flowing in remove-unused-brs (#1639)

The fuzzer found a bug with flowing of values in that pass: when one arm of an if is none-typed, we can't flow a value through the other. Odd the fuzzer didn't find this earlier, as it's been a bug since the pass was written years ago, but in practice it seems you need a specific set of circumstances on the outside for it to be hit.

The fix is to stop flowing a value in that case. Also, I realized after fixing it that the valueCanFlow global state variable is entirely unneeded. Removing it makes the pass significantly simpler: at all times, flows contains branches and values that might be flowing, and if the flow stops we remove them, etc. - we don't need an extra state variable to say if flowing is possible. So when we want to use the flows, we just check what is there (and then for a flowing branch we can remove it, and for a flowing value we can replace the branch with the value, etc., as in both cases they flow to the right place anyhow).

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