This change allows spriv-reduce to get rid of a selection, switch or
loop construct if none of the instructions defined in the construct
are used outside the construct.
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.
* 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.
