[Constant] Add containsPoisonElement

This patch

- Adds containsPoisonElement that checks existence of poison in constant vector elements,
- Renames containsUndefElement to containsUndefOrPoisonElement to clarify its behavior & updates its uses properly

With this patch, isGuaranteedNotToBeUndefOrPoison's tests w.r.t constant vectors are added because its analysis is improved.

Thanks!

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D94053