[VPlan] Add splitAt unit test. NFC (#164636)
@sink_replicate_region_4_requires_split_at_end_of_block was originally
added to ensure splitting at the end of a block wouldn't crash, see
bdada7546e6b4a189a22c7ba9ce2d1b507b9c22e
However it looks like we're now no longer testing this because conv
isn't at the end of the block anymore.
This moves it into a unit test instead. Discovered when working on
https://github.com/llvm/llvm-project/pull/160449
