Reapply r198478 "Fix PR18361: Invalidate LoopDispositions after LoopSimplify hoists things."

Now with a fix for PR18384: ValueHandleBase::ValueIsDeleted.

We need to invalidate SCEV's loop info when we delete a block, even if no values are hoisted.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198631 91177308-0d34-0410-b5e6-96231b3b80d8
Fix c++ mode comments

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198632 91177308-0d34-0410-b5e6-96231b3b80d8