[MemorySSA] Call the correct dtors

It appears that there were many cases where we were directly (through
templates) calling the dtor of MemoryAccess, which is conceptually an
abstract class.

This hasn't been a problem, since the data members of all of the
subclasses of MemoryAccess have been POD. I'm planning on changing that.
:)

llvm-svn: 326175