Use backward layer for computing yRev even when an initial state is given.

[adarob]

This change is 

[bileschi]

Thanks @adarob , it would have been nice if this bug had been exposed in a test. Would it be possible / difficult to add a test that would be broken but for your fix?

[caisq]

@adarob, +1 to what @bileschi said. If you are willing to add the test that would be great. We'll also be happy to merge this PR first and add a test ourselves. Just let us know. Thanks.

[caisq]

@bileschi Given we are planning a new minor release soon, I'll merge this PR first and add tests for it later. How does that sound?

[bileschi]

@caisq works for me.

[caisq]

@bileschi Thanks.