[PM] Add a unittest for invalidating module analyses with an SCC pass.

This wasn't really well explicitly tested with a nice unittest before.
It seems good to have reasonably broken out unittests for this kind of
functionality as I'm workin go other invalidation features to make sure
none of the existing ones regress.

This still has too much duplicated code, I plan to factor that out in
a subsequent commit to use common helpers for repeated parts of this.

llvm-svn: 280447