[MachineOutliner][NFC] Simplify isMBBSafeToOutlineFrom check in AArch64 outliner

Turns out it's way simpler to do this check with one LRU. Instead of
maintaining two, just keep one. Check if each of the registers is available,
and then check if it's a live out from the block. If it's a live out, but
available in the block, we know we're in an unsafe case.

llvm-svn: 346721