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.
* Run validator during reduction.
* Added functionality to validate modules after each reduction step, and some tests to check this is working. Also fixed an issue where reduction passes were not guaranteed to be executed at their minimum granularities.
