[IPConstantProp][NFCI] Improve and modernize tests
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
Differential Revision: https://reviews.llvm.org/D69747