VectorCombine: add tests written for InstSimplify (#92776)

21419071 (InstSimplify: increase shufflevector test coverage) was
recently merged as a pre-commit test for some work that was misguided.
It turns out that InstSimplify can never work on those tests, but the
tests are useful nevertheless; move them to VectorCombine to support the
development of VectorCombine::foldShuffleToIdentity.