[llvm] [VPlan] Add a test for testing unused interleave recipes (PR #75026)

[Florian Hahn via llvm-commits]

https://github.com/fhahn approved this pull request.

LGTM, but please see if the block names in unused-interleave-group.ll could also be cleaned up before landing as well of adding a `vplan-` prefix to the test file name.



https://github.com/llvm/llvm-project/pull/75026