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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268590 91177308-0d34-0410-b5e6-96231b3b80d8