Added DEBUG messages to the top of several processing loops in ObjCARC.cpp that emit what instructions are being visited.

This is a part of a larger effort of adding DEBUG messages to the ARC
Optimizer Backend.
Added DEBUG message when ObjCARC replaces a call which returns its argument verbatim with its argument to temporarily undo an optimization.

Specifically these calls return their argument verbatim, as a low-level
optimization. However, this makes high-level optimizations
harder. We undo any uses of this optimization that the front-end
emitted. We redo them later in the contract pass.