ARM: Use a Handle to track SDNodes in case they're CSE'd. NFC

The code here is recursively Select-ing a new Node to avoid issues
where N is CSE'd during replaceDAGValue and stops being valid. We can
accomplish the same goal in a more principled way by using a
HandleSDNode.

This is essentially a less dodgy fix for PR25733 than the original
attempt back in r255120.

llvm-svn: 268590