[MSSA] Allow unittests to use BasicAA when building.

We now build MemorySSA in its ctor, instead of waiting until the user
calls MemorySSA::getWalker. This silently changed our unittests, since
we add BasicAA to AAResults *after* constructing MemorySSA (...but
before calling MemorySSA::getWalker).

None of them broke because we do most of our "did this get optimized
correctly?" tests in .ll files.

llvm-svn: 283158