[InstSimplify] Remove select ?, undef, X -> X and select ?, X, undef -> X transforms

As noted here https://lists.llvm.org/pipermail/llvm-dev/2016-October/106182.html and by alive2, this transform isn't valid. If X is poison this potentially propagates poison when it shouldn't.

This same transform still exists in DAGCombiner.

Differential Revision: https://reviews.llvm.org/D83360