[llvm][PPCISelDAGToDAG] rename ppc-codegen to ppc-isel

Every other subclass of SelectionDAGISel calls this pass "<arch>-isel".
No existing tests refer to ppc-codegen so this is purely a cosmetic
change to bring the pass name in line with other architecture's
SelectionDAGISel subclasses.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D140497