Introduce [post]dominator tree and related infrastructure, use it in CFG func

verifier.  We get most of this infrastructure directly from LLVM, we just
need to adapt it to our CFG abstraction.

This has a few unrelated changes engangled in it:
 - getFunction() in various classes was const incorrect, fix it.
 - This moves Verifier.cpp to the analysis library, since Verifier depends on
   dominance and these are both really analyses.
 - IndexedAccessorIterator::reference was defined wrong, leading to really
   exciting template errors that were fun to diagnose.
 - This flips the boolean sense of the foldOperation() function in constant
   folding pass in response to previous patch feedback.

PiperOrigin-RevId: 214046593