[Attributor][NFCI] Introduce tight iteration bounds in the tests

Summary:
To be able to track how many iterations we need to manifest all
information we check for we now make the maximum iteration count
explicit. The count is set tightly now and should be kept that way.

Reviewers: uenoku, sstefan1

Subscribers: bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66554

llvm-svn: 369586