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

[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*`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273189 91177308-0d34-0410-b5e6-96231b3b80d8