[Attributor] Deal with shrinking dereferenceability in a loop

[Attributor] Deal with shrinking dereferenceability in a loop

Summary:
If we have a loop in which the dereferenceability of a pointer decreases
we did slowly decrease it iteration by iteration, leading to a timeout.
With this patch we detect such circular reasoning and indicate a
fixpoint early.

Reviewers: uenoku, sstefan1

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66558

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369784 91177308-0d34-0410-b5e6-96231b3b80d8