AArch64: Stop using MachineInstr::getNextNode()

Stop using `getNextNode()` to get an insertion point (at least, in this
one place).  Instead, use iterator logic directly.

The `getNextNode()` interface isn't actually supposed to work for
creating iterators; it's supposed to return `nullptr` (not a real
iterator) if this is the last node.  It's currently broken and will
"happen" to work, but if we ever fix the function, we'll get some
strange failures in places like this.

llvm-svn: 249764