[InstCombine] Update instcombine for vectorOps to use new shufflevector semantics

This patch updates the transformations in InstCombineVectorOps to use the new
hufflevector semantics that say that undefined values in the mask yield poison.

To prevent miscompilations we have to match with m_Poison instead of m_Undef.
Otherwise, we might introduce poison where there was previously undef.

Differential Revision: https://reviews.llvm.org/D150039