[Attributor] Reach optimistic fixpoint in AAValueSimplify when the value is constant or undef

Summary:
As discussed in D71799, we have found that it is more useful to reach an optimistic fixpoint in AAValueSimpify when the value is constant or undef.

Reviewers: jdoerfert, sstefan1

Reviewed By: jdoerfert

Subscribers: baziotis, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71852