[InstSimplify] Don't fold vectors of partial undef in SimplifySelectInst if...

[InstSimplify] Don't fold vectors of partial undef in SimplifySelectInst if the non-undef element value might produce poison

We can't fold to the non-undef value unless we know it isn't poison. So check each element with isGuaranteedNotToBeUndefOrPoison. This currently rules out all constant expressions.

Differential Revision: https://reviews.llvm.org/D83442