ADT: Remove external uses of ilist_iterator, NFC

Delete the dead code for Write(ilist_iterator) in the IR Verifier,
inline report(ilist_iterator) at its call sites in the MachineVerifier,
and use simple_ilist<>::iterator in SymbolTableListTraits.

The only remaining reference to ilist_iterator outside of the ilist
implementation is from MachineInstrBundleIterator.  I'll get rid of that
in a follow-up.

llvm-svn: 280565