[Attributor][NFC] Invalid DerefState is at fixpoint

Summary:
If the DerefBytesState (and thereby the DerefState) is invalid, we
reached a fixpoint for the whole DerefState as we will not
manifest/provide information then.

Reviewers: uenoku, sstefan1

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65586

llvm-svn: 367789