Visit composite transform nodes in Pipeline._check_replacement()

Pipeline.replace_all() calls _replace(), which replaces transform nodes using a PipelineVisitor, and then _check_replacement(), which validates that there are no replaceable transform nodes using a PipelineVisitor i.e. it checks that PTransformOverride.matches() does not return True on any nodes.

Currently, _replace() visits [both composite and leaf transform nodes](https://github.com/apache/beam/blob/b7ae7bb1dc28a7c8f26e9f48682e781a74e2d3c4/sdks/python/apache_beam/pipeline.py#L376-L382) whereas _check_replacement() visits [only leaf transform nodes](https://github.com/apache/beam/blob/b7ae7bb1dc28a7c8f26e9f48682e781a74e2d3c4/sdks/python/apache_beam/pipeline.py#L463-L468). _check_replacement() should visit composite transform nodes as well for symmetry.

Imported from Jira [BEAM-11024](https://issues.apache.org/jira/browse/BEAM-11024). Original Jira may contain additional context.
Reported by: myffical@gmail.com.