PM: Check that loop passes preserve a basic set of analyses

PM: Check that loop passes preserve a basic set of analyses

A loop pass that didn't preserve this entire set of passes wouldn't
play well with other loop passes, since these are generally a basic
requirement to do any interesting transformations to a loop.

Adds a helper to get the set of analyses a loop pass should preserve,
and checks that any loop pass we run satisfies the requirement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268444 91177308-0d34-0410-b5e6-96231b3b80d8