[PM] Teach LVI to correctly invalidate itself when its dependencies

[PM] Teach LVI to correctly invalidate itself when its dependencies
become unavailable.

The AssumptionCache is now immutable but it still needs to respond to
DomTree invalidation if it ended up caching one.

This lets us remove one of the explicit invalidates of LVI but the
other one continues to avoid hitting a latent bug.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292769 91177308-0d34-0410-b5e6-96231b3b80d8