`DecomposeCallGraphTypes.cpp` was a workaround around missing 1:N
support in the dialect conversion. Now that 1:N support was added, the
workaround can be deleted. The test remains in place, as an example for
how to write such a transformation with the dialect conversion
framework.
Note for LLVM integration: If you are using
`DecomposeCallGraphTypes.cpp`, switch to the patterns that are used in
`TestDecomposeCallGraphTypes.cpp`.
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