[ARM][LowOverheadLoops] Update liveness info

Recommitting e93e0d41 after reverting due to test failures, which
will hopefully now be fixed. Original commit message:

After expanding the pseudo instructions, update the liveness info.
We do this in a post-order traversal of the loop, including its
exit blocks and preheader(s).

Differential Revision: https://reviews.llvm.org/D72131