[MemorySSA] Clean up unit tests a tiny bit. NFC.

We recently made MemorySSA own the walker it creates. As a part of this,
the MSSA test fixture was changed to have a `Walker*` instead of a
`unique_ptr<Walker>`. So, we no longer need to do `&*Walker` in order to
get a `Walker*`.

llvm-svn: 273189