[llvm] [SelectionDAG] Introducing a new ISD::POISON SDNode to represent the poison value in the IR. (PR #135056)

[zhijian lin via llvm-commits]

diggerlin wrote:

I updated the patch with  the original PR (https://github.com/llvm/llvm-project/pull/125883)  and  the  fix for  " [DAGTypeLegalize] Legalize ISD::POISON as ISD::UNDEF for Float Type Legalization" , and change the title and summary. 

https://github.com/llvm/llvm-project/pull/135056