This patch adds a simplistic backend that gathers all target-specific
SelectionDAG nodes and emits descriptions for most of them.
This includes generating node enumeration, node names, and information
about node "prototype" that can be used to verify that a node is valid.
The patch also extends SDNode by adding target-specific flags, which are
also included in the generated tables.
Part of #119709,
[RFC](https://discourse.llvm.org/t/rfc-tablegen-erating-sdnode-descriptions/83627).
Pull Request: https://github.com/llvm/llvm-project/pull/123002
6aeffcdb91