Roundtrip the undef node

[halvorlinder]

This PR roundtrips the undef node and provides a unit test for it.
In addition, it updates the commit hash used for building the mlir-rvsdg dialect to include the fix of the undef operation output type.

[phate]

Only minor comments

[sjalander]

@phate @haved This test uses both the backend and frontend.
I suggest that we place it in test/jlm/mlir instead of either the backend or frontend.
The name of the file could be MlirRoundtripTest.cpp or JlmToMlirToJlmTest.cpp.

[phate]

@sjalander Alternatively, we could split the test in two: one for frontend and one for backend?

[sjalander]

@phate In this case we could, but that adds extra work as one then need to manually construct both the RVSDG and MLIR graph for the two individual tests. That's what we have done so far, but it is tedious work that doesn't contribute to improving the test coverage.

This wouldn't work in general for the roundtrip tests we have talked about, where one create an RVSDG graph that gets converted to MLIR and then back to RVSDG-new and a simple comparison between RVSDG and RVSD-new is made.
Such tests have the added benefit that they ensure that the backend and frontend generates consistent results.

[phate]

Place them in test/jlm/mlir then.