[MemorySSA] Port to new pass manager

Add support for the new pass manager to MemorySSA pass.

Change MemorySSA to be computed eagerly upon construction.

Change MemorySSAWalker to be owned by the MemorySSA object that creates
it.

Reviewers: dberlin, george.burgess.iv

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19664

llvm-svn: 271432