[IPConstantProp][NFCI] Improve and modernize tests

Summary:
This change is in preparation to reuse these test for the Attributor.
It mainly is to remove UB, make it clear what is tested, and use
"modern" run lines.

Reviewers: fhahn, efriedma, mssimpso, davide

Subscribers: bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69747