PowerPC: Don't use getNextNode() for insertion point

Stop using `getNextNode()` to create an insertion point for machine
instructions (at least, in this one place).  Instead, use an iterator.
As a drive-by, clean up dump statements to use iterator logic.

The `getNextNode()` interface isn't actually supposed to work for
insertion points; it's supposed to return `nullptr` 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.

llvm-svn: 249758