[llvm][AArch64ISelDAGToDAG] support -{start|stop}-{before|after}=aarch64-isel

Follow a similar pattern as AMDGPUDAGToDAGISel's constructor so that we
can use INITIALIZE_PASS to register a pass. This allows for more fine
grain testability of SelectionDAGISel via:

   llc -stop-{before,after}=aarch64-isel

Link: https://github.com/llvm/llvm-project/issues/59538

See also: https://reviews.llvm.org/D140323

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D140331