[MSSA] Delete move ctor; remove dynamic never-moved verification
Code archaeology in D59315 revealed that MSSA should never be moved.
Rather than trying to check dynamically that this hasn't happened in the
verify() functions of Walkers, it's likely best to just delete its move
Since all these verify() functions did is check that MSSA hasn't moved,
this allows us to remove these verify functions.
I can readd the verification checks if someone's super concerned about
us trying to memcpy MemorySSA or something somewhere, but I imagine we
have other problems if we're trying anything like that...