[Attributor] Do not update or manifest dead attributes

Summary:
If the associated context instruction is assumed dead we do not need to
update or manifest the state.

Reviewers: sstefan1, uenoku

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66116

llvm-svn: 368921