[Attributor] Reuse the IPConstantProp tests for the Attributor

The Attributor can, to some degree, do what IPConstantProp does. We can
consequently use the corner cases already collected and tested for in
the IPConstantProp tests to improve Attributor test coverage.

This exposed various bugs fixed in previous Attributor patches.

Not all functionality of IPConstantProp is available in AAValueSimplify
and AAIsDead so some tests show that we cannot perform the expected
constant propagation.

Reviewers: fhahn, efriedma, mssimpso, davide

Subscribers: bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69748