MLIR's C++ `Operation::walk` supports type-filtered traversal (e.g.
`op->walk([](arith::AddIOp op) { ... })`), but the Python binding
`op.walk()` requires users to manually implement type filtering inside
the callback function.
This PR adds type filtering into the python binding `op.walk()`, if
users pass `op_class`, walk() will only apply callback to matching ops.
This PR also adds a common use helper in mlir/ir that collects all ops
of a given type into a list. Users can just call: `ops =
ir.get_ops_of_type(root, op_class)`.
39b3b2e0e8