[MLIR][GPU] Add debug output to enable dumping GPU assembly

- Set the DEBUG_TYPE of SerializeToBlob to serialize-to-blob
- Add debug output to print the assembly or PTX for GPU modules before
  they are assembled and linked

Note that, as SerializeToBlob is a superclass of SerializeToCubin and
SerializeToHsaco, --debug-only=serialize-to-blom will dump the
intermediate compiler result for both of these passes.

In addition, if LLVM options such as --stop-after are used to control
the GPU kernel compilation process, the debug output will contain the
appropriate intermediate IR.

Reviewed By: herhut

Differential Revision: https://reviews.llvm.org/D117519