[IR] Let IRBuilder's CreateVectorSplat/CreateShuffleVector use poison as placeholder

This patch updates IRBuilder to create insertelement/shufflevector using poison as a placeholder.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D93793

GitOrigin-RevId: 278aa65cc495b2f548018eb2fd96351d36b4591f
4 files changed