[llvm][X86ISelDAGToDAG] support -{start|stop}-{before|after}=x86-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.

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

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D140323