CodeGen: Assert that bundle iterators are valid

Add an assertion to the MachineInstrBundleIterator from instr_iterator
that the underlying iterator is valid.  This is possible know that we
can check ilist_node::isSentinel (since r281168), and is consistent with
the constructors from MachineInstr* and MachineInstr&.

Avoiding the new assertion in operator== and operator!= requires four
(!!!!) new overloads each.

(As an aside, I'm strongly in favour of:
- making the conversion from instr_iterator explicit;
- making the conversion from pointer explicit;
- making the conversion from reference explicit; and
- removing all the extra overloads of operator== and operator!= except
  const_instr_iterator.

I'm not signing up for that at this point, but being clear about when
something is an MachineInstr-iterator (possibly instr_end()) vs
MachineInstr-bundle-iterator (possibly end()) vs MachineInstr* (possibly
nullptr) vs MachineInstr& (known valid) would surely make code
cleaner... and it would remove a ton of boilerplate from
MachineInstrBundleIterator operators.)

llvm-svn: 281170