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.
* Add OperandToUndefReductionPass.
Fixes#2115.
Also added some tests that are similar to those in OperandToConstantReductionPassTest.
In addition, refactor FindOrCreateGlobalUndef into reduction_util.cpp. Fixes#2184.
Removed many documentation comments that were identical or very similar to the overridden function's documentation comment.
