Move BB succ_iterator to be inside TerminatorInst. NFC.

To get the successors of a BB we currently do successors(BB) which
ultimately walks the successors of the BB's terminator.

This moves the iterator to TerminatorInst as thats what we're actually
using to do the iteration, and adds a member function to TerminatorInst
to allow us to iterate directly over successors given an instruction.

For example, we can now do

  for (auto *Succ : BI->successors())

instead of

  for (unsigned i = 0, e = BI->getNumSuccessors(); i != e; ++i)

Reviewed by Tobias Grosser.

llvm-svn: 244074