[MemorySSA] Add APIs to move memory accesses between blocks, following CFG changes.

[MemorySSA] Add APIs to move memory accesses between blocks, following CFG changes.

Summary:
The move APIs added in this patch will be used to update MemorySSA when CFG changes merge or split blocks, by moving memory accesses accordingly in MemorySSA's internal data structures.
[Split from D45299 for easier review]

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D48897

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336860 91177308-0d34-0410-b5e6-96231b3b80d8