[LV] Weaken spuriously strong assert in LoopVersioning

LoopVectorize uses some utilities on LoopVersioning, but doesn't actually use it for, you know, versioning.  As a result, the precondition LoopVersioning expects is too strong for this user.  At the moment, LoopVectorize supports any loop with a unique exit block, so check the same precondition here.

Really, the whole class structure here is a mess.  We should separate the actual versioning from the metadata updates, but that's a bigger problem.