Refactor the 'walk' methods for operations.

This change refactors and cleans up the implementation of the operation walk methods. After this refactoring is that the explicit template parameter for the operation type is no longer needed for the explicit op walks. For example:

    op->walk<AffineForOp>([](AffineForOp op) { ... });

is now accomplished via:

    op->walk([](AffineForOp op) { ... });

PiperOrigin-RevId: 266209552