Address review feedback.

- LangRef.rst: missing ':', stale example
- LLParser.cpp: auto *
- Bitcode: remove ptrty/addrdiscty
- AsmWriter.cpp: ListSeparator
- Constants.h: cast<>
- Constants.cpp: stale NDEBUG
- Constants.cpp: stale comments
- Constants.cpp: use getTypeSizeInBits for offset accumulation,
  to allow pointer/integer types without fiddling.
- ConstantFold.cpp: remove FIXME, should be addressed separately
- ValueTracking.cpp: remove from isGuaranteedNotToBeUndefOrPoison