Revert "[SLP] Optionally preserve MemorySSA"

This reverts commit 1cfa986d.  See https://github.com/llvm/llvm-project/issues/54256 for why I'm discontinuing the project.

Seperately, it turns out that while this patch does correctly preserve MSSA, it's correct only at the end of the pass; not between vectorization attempts.  Even if we decide to resurrect this, we'll need to fix that before reapplying.