[ARM] Simplification to ARMBlockPlacement Pass.

It simplifies the logic by moving the predecessor  (preHeader or it's predecessor) above the target (or loopExit),
instead of moving the target to after the predecessor.

Since the loopExit is no longer being moved, directions of any branches within/to it are unaffected.

While the predecessor is being moved, the backwards movement simplifies some considerations,
and the only consideration now required is that a forward WLS to the predecessor should not become backwards.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D100094