[MemorySSA] Ensure address stability of MemorySSA object.

Summary:
Ensure that the MemorySSA object never changes address when using the
new pass manager since the walkers contained by MemorySSA cache pointers
to it at construction time.  This is achieved by wrapping the
MemorySSAAnalysis result in a unique_ptr.  Also add some asserts that
check for this bug.

Reviewers: george.burgess.iv, dberlin

Subscribers: mcrosier, hfinkel, chandlerc, silvas, llvm-commits

Differential Revision: https://reviews.llvm.org/D23171

llvm-svn: 278028