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

[Florian Hahn via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
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fhahn wrote:

nit: I think the existing convention is to prefix tests checking VPlan output only with `vplan-`

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