AArch64: Make getNextNode() cleanup in r249764 more clear

After r249764, if you didn't see the full context, it looked like
`std::next(I)` would get the same result as
`++MachineBasicBlock::iterator(I)`.  However, `I` is a `MachineInstr*`
(not a `MachineBasicBlock::iterator`).

Use the `getIterator()` helper I added later (r249782) to make this code
more clear.

llvm-svn: 249852