[PM] Teach SCEV to invalidate itself when its dependencies become

[PM] Teach SCEV to invalidate itself when its dependencies become
invalid.

This fixes use-after-free bugs that will arise with any interesting use
of SCEV.

I've added a dedicated test that works diligently to trigger these kinds
of bugs in the new pass manager and also checks for them explicitly as
well as triggering ASan failures when things go squirly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291426 91177308-0d34-0410-b5e6-96231b3b80d8