Add some val options to reduce (#2401)

Fix #2396 

* Check that initial state is valid. Add kInitialStateInvalid. 
* Fix RemoveOpnameAndRemoveUnreferenced test; turns out the original shader is invalid, but we never notice because we don't check this and the reduced shader is valid; fix original shader. Assert reduction status is kComplete.
* Always check return value from `Reducer::Run`.
* Change Reducer::Run to *not* immediately copy the input binary.