[SLP] Optionally preserve MemorySSA

This initial patch adds code to preserve MemorySSA through a run of SLP vectorizer. The eventual plan is to use MemorySSA to accelerate SLP's memory dependence checking, but we're a ways from that.  In particular, this patch is correct, but really slow. It's being landed so that we can work incrementally in tree, not because it's expected to be useful to anyone just yet.

The broader effort is being tracked in https://github.com/llvm/llvm-project/issues/54256.  Its worth noting expicitly that this may not work out, and if not, we will be reverting all of the MSSA support in SLP at some point in the next few weeks.

Differential Revision: https://reviews.llvm.org/D117926