[LVI] Add some additional comments about caching and context instructions

Philip Reames and I had a long conversation about this, mostly because it is
not obvious why the current logic is correct. Hopefully, these comments will
prevent such confusion in the future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219882 91177308-0d34-0410-b5e6-96231b3b80d8