GlobalISel already handles undefined workitem.id.{x,y,z} intrinsics,
SelDAG failed in AMDGPUISelLowering.cpp due to a failed assertion in
`AMDGPUTargetLowering::loadInputValue`: `Arg && "Attempting to load
missing argument"`. This commit changes the behavior of SelDAG to
instead use a zero constant.
This LLVM defect was identified via the AMD Fuzzing project.
bcba3117c0