Fix#2475. Fix#2476.
* Improve reducer algorithm: shrink granularity, remove an early return, no lazy initialization, notify pass if binary is interesting, add comments.
* Add fail-on-validation-error option to fail a reduction if an invalid state is reached; useful for tests.
* Set fail-on-validation-error in tests.
* Improve some documentation comments.
* Add Reducer::AddDefaultReductionPasses so tests (and other library consumers) can add the default reduction passes.
* Add CLIMessageConsumer in test_reduce so we can see messages for tricky tests.
* Remove test RemoveUnreferencedInstructionReductionPassTest_ApplyReduction because it was indirectly testing the reduction algorithm, not the RemoveUnreferencedInstruction pass.
* Tweak tests where needed.
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.
